Knee-Up Update

Alright, so I have a confession. I haven't been able to keep up on increasing the number of knee-ups I can do. I've been doing them and pull-ups nearly everyday, but my progress forward has ceased for the time being. I've just finished day 13 of the 21 day challenge, and I'm still resting at 10 knee-ups. I'm hoping for this to increase soon! But I am only pushing my body as hard as I can right now, and its actually my arms that are struggling here. Any suggestions?