Fastpasses DO NOT expire. I have spoken with a family member who is in upper management at WDW. Again, they DO NOT expire. If you have a problem, politely ask for a supervisor. Many ride operators do not know this policy and it never came up during training. Apparently, this is becoming an issue and new cast members are being spcifically trained on this. Did I mention that Fastpasses DO NOT expire.
I know that I had a CM tell me that they did not expire.