Fluoxetine stopped working on my cat

I have a call into the vet, but wanted to post here as well. My cat was spraying (urinating) against a wall. We started him on fluoxetine mid April. He had stopped the spraying until about 2 weeks ago..It has begun again. Never misses a dose and there have been absolutely no changes to our home. Same food, same litter etc. Does this happen? I don't know what else to try. We tried Feliway for months before the meds and that did nothing. I wanted so badly to have that room redone (where he had been urinating) but no sense doing that if he will continue.