lol, its prob a mental block. Dont count your own pushups and just keep counting 1, 1, 1, 1 over and over in your head. Have a buddy count silently while you do that. Do it until you cant do anymore. You will prob do at least 10 more.
When I was in the Marines I could never get to 20 pullups because in my mind I knew I would get tired around 15. I stopped counting them out and was doing 17-18 after that. The minds a powerful thing.