I do slow push-ups (an attempt at going easy on my wrists) - I hold position at the lowest point of the push-up for three seconds and then back up again. One thing I've found to make a large difference in how much I can do is the breathing technique: In the slow push-up, once I've reached the lowest point, I exhale slowly through the three seconds, then when pushing back up again, I'm filling lungs to capacity. As I understand it, emptying lungs completely is an important factor as well as choosing a breathing pattern which actually helps the exercise.
With this breathing technique I do a lot better than if I start exhaling while pushing down, for example. I'm not going to claim that I've got it figured out, but this seems to be what works for me.
One other thing though, bear in mind that sometimes you're ahead, sometimes you're behind, the important thing is consistent exercise which gives your muscles a workout. With my slow push-ups I'm typically averaging about 15 a day (in one sitting), but if I only manage say 12 then so be it. Another day I'll push to say 17 because it feels right. Don't get worked up just because you plateaued a bit (which is easy advice to give and harder to follow).