There is only one reason I can think of.
I, once, uploaded like a million brushes on PS6, and, when I reopened PS6 they all ran away from me, apparently. After a few times, I noticed it was the amount of brushes I had placed up (not sure why).
What I would suggest is put up like one or two brushes to the list, and see if they are still there when you reopen Photoshop.
Hope this helps.