I don't know if it can stop working. I have only taken provera twice. Once was a long time ago - like 10 years or so - and I don't remember how it worked. I just did 6 days' worth recently (Nov 15-20) and had some bloody streaking beginning Nov 24. But it was like red streak on the TP... occasional reddish goop coming out while on the toilet... some more streaks on the TP... nothing that preogressed to what I would call a real period. Today there seems to be nothing going on.
A couple months ago I induced AF with 5 days of BCPs... a few days after stopping I had some pink on the TP for a few days... ocasional pink gob in the toilet... no real period. But then 2 weeks after stopping, AF came on for real.
I am hoping I am going to see something similar this time and that I just need to wait a little longer.
I don't know what to tell you if Provera has always worked more quickly for you, though. I wouldn't think it could stop working... your body doesn't stop responding to its own progesterone over time, women generally don't stop responding to BCPs over time... you know?
Maybe you've just been bleeding here and there all along and now there's not a big lining to be shed?
I don't know.