it seems that everyone runs into this problem. I think that once people start finding out that they have to do these horrible creations called pullups, they start working on them hard core. Then after some time, the armstrong and sets up to 50, etc. just sort of stop working. You hit a slump. My slump was first at 12, then at 17. Honestly I think that you just need to keep doing what you're doing. The "slump" will end eventually. (Unless of course you're doing stupid stuff like hurting yourself). I would just keep up the work you've been doing now. You cant go from 10 to 20 in a month. Just keep doing them...a lot