Shooting Blanks??

I have a 6 year old neutered male cat. He still is into humping things like I've seen that many others are. I never really thought anything about it, but just a little bit ago I noticed that he was humping the couch. I moved him so he would stop (which he did not like) and I noticed that he left a tiny spot. Maybe I am uneducated about how all this works but if he is neutered, should he be able to do that? Is he just "shooting blanks" or what?

You got me! Sometimes the surgeries get botched. Maybe that's the problem? You might want to check with a vet just in case.