My Answer:

First let's set some ground rules of understanding concerning the Name and Names of our Creator -- the God and orignialauthor of Torah (first five books of the Old Testament):  The Creator and God of Israel has a Name and this Name is composed of four letters and these four letters are called the Tetragrammaton (This term is actually from the Greek, menaing four).  This term is used by biblical scholars in reference to the four letters Y-H-W-H,  these letters represent the Hebrew characters in English.  In ancient Hebrew (Paleo Hebrew) the "four letters" appear differently from what we see in "four letters" of modern Hebrew.  These "four letters" have been replaced by the scribes and tranlators of scripture with the tilte "LORD" in capital letters. 

They, the scribes, have exchanged the "four letters" of our Creator's Name for a different set of four letters, in effect, changing His Name, the Name of the God of Israel.  YHWH, pronouced as YaHWeH,, not Yeweh.  With this understood we should be able to determine way we do not fine the Name of our Savior in the Torah spelled as YaHshua but we do find it spelt as Yeshua and an even more correct, YeHoshua.  The "four letter" Name, YHWH, pronounciation is usually avoided by those responsible for the copying of the Torah (Hebrew Bible).   Not all of the Jewish scholars believe this, but there is a majority of practicing Jews believe the "four letters" are too sacred to mention and protect this by using, or saying "adonay" (spelling of this term varies).  This term means "Lord, or Master", the same definition as LORD.  YHWH does not mean "Lord" or Master" but IS Master.  YHWH is literally, YH is Creator.  YH, as personal Name of our Creator is indefinable and is in reference to the Creator, period.

The first two letters of the Tetragrammaton (YH) is found in the Savior's birth Name -- anything between these first two letters is a vowel, supplied y the translator, or scribe, and not part of the actual Name.  Vowels act as pronounciation aid and is not part of the Sacred Name in its purist form.  Some wish to use the pronounciation Yeshua as this is what is found in the Torah, as copied by the scribes. It should be realized, the scribes, the copyist and the publishing houses add the vowel "e" at their own discretion.  You will notice the obvious, the second letter of the tetragrammaton, "H", has been dropped and replaced with the "e" vowel. 

If this were a mystrey novel one might call this "The"Case of the Missing Letter", but in this case it is not a mystery, men have decided, in their own wisdom,  to drop part of our Savior's Name. The Name of our Savior  is weakened by the dropping a letter from from His Name, a letter that points toward His Father, His Birth name Name.  the Christian community have gone along with this as it fits nicely along side the accepted bogus name "Jesus".  In many preface at the front of Bible translations you will find tyhe excuse for why the translators have decided not to use the Sacred Name of the God of Israel -- in most cases they say they are following the "traditions of the fathers", you know, the translators before them.  Ultimately, this "tradition" comes down to the Christian translators from the Jewish scribes.  So, the ground rule to recognize is:  The Tranlslators on both side of the aisle have decided not to use the true identity of the Creator of all the things, refusing, in their wisdom, to also purge the Father's Name from that of His Son.  The Christians have gone the Jewish scribes one better -- they have change the Name and the true identity of our Savior completely to somthing else.  Realize this one rule and you wil begin to see through the deception, the lie that it does not matter.

So, the answer to the question is, the reason you do not find "YaHshua" but you do find Yeshua and YeHoshua is -- the scribes and translators decided to use the "e" vowel instead of the "a" vowel and in one case decided to drop part of the Name altogether by leaving the second letter of the Creator's name on the floor, so to speak.  In their efforts to treat the Name as too sacred to use they have "profaned" it by making useless, by hidding it.  The Savior's Name is found in the Hebrew scriptures, as your question suggests, in two different forms and it is a small argument to worry over a "vowel", of being "one point" to the left or to the right.  When being pronounced, wether the "a" vowel or the "e" vowel, the Name itself is maintained as YH, and YHWH -- you pronounce it, forget about what the scribes have done.  The travisty is what the Christians have done, they acknowledge the God of Israel, for the most part, as YHWH (YaHWeH) but refuse the Savior's Identiy, replacing it with a different name intirely.  They have one-upped the Hebrew scholars.  

It is not Hidden

The name "Joshua" is in the Bible and that is the very Name our Savior was Named at birth.  Who has not heard of the Book of Joshua?  It is seen or printed in the Torah as YeHoshua, being translated, over the centuries by the English as "JOSHUA", but not in the New Testament -- it is strangely changed to a false name, JESUS -- and name more in common with the Roman Latin, or convoluted Greek distortion, IEOUS.  Anything but His birth Name as deliverd by the Messanger to both Mary and Joseph (Mat 1:21)

 In the Latin and Greek language the letter "s" is their letter of ownership.  Roman and Greek names are notorious for finishing in the letter "s" or "us" .  With the Hebrews it is the letter "a", or of the "a" sound.  This can be explained as a type of dialect, setting the sound of one language apart from another.  The vowel letters most favorable to the Hebrew scribes is a, o, and u. The vowel letters most favored by the Romans and Greeks -- e, u, s.  Isn't it strange how a name that is supposed to have the short form of the Sacred Name in it is missing a primary letter as seen in the Torah in the name Yeshua, as opposed to YeHshua?  Isn't it strange the vowel used in place of this missing letter is an e when it is obvious, from a Jewish point of view, it should be the "a" vowel.  When the Torah does produce the Name matching the Name given to our Savior, restoring the "H" to its proper place they still supply the "e" over the "a" as the vowel choice, following the Greek scribes -- this is incredible.