Fluoxetine stopped working on my cat

I know urine spraying is very frustrating. I'm afraid the true reason he started spraying again in that area may be a bit of a mystery known only to your cat. Fluoxetine is certainly the best behavioral medication to start initially for inappropriate urination. Now, if he does go back to that same spot every time (and it's not a variety of areas he's going on) there is always the issue of the urine soaking into the wall or floor surface. This is almost impossible to get rid of with cleaners and refinishing is often needed, unless you can reliably cover the area with something he's unlikely to pee on. 

My family once lived in a house where we had a huge problem with my cats spraying in along the wall/carpet junction in the dining room (and nowhere else). Turns out the owners who we were renting from and lived there previously had cats in the home that sprayed there before. It just goes to show how long that can linger. After we moved to a new home, they didn't continue the behavior.

If it is just one area, it may be worth actually refinishing the area if that work would involve removing any urine-soaked or stained components. Increasing the fluoxetine dose may be another option you could talk to your vet about. But there is no way to know if that will get the result you want. There are other medication options that could be considered too.