When I attended the 68V class the only Soldiers that fell off of ALI were the ones that didn't pass a PT test for the roughly a year they were at Ft Sam. If this does not apply to you then it could be that you never updated your passing PT test. Check your ERB and if you don't have a current one from at least the past year, this is why.
Unfortunately you may have been in a unit that spoiled you by updating the PT test automatically.....this is actually how it should be.
What I have seen in actuality is that the Soldier may not necessarily know that it actually is their responsibility to ensure it is updated.
How has this been playing out for you anyways?