i can rep out the sets without a problem, its the quality of the rep that i am not happy with. Also the volume falls away significantly. usually something like 10 8 5 on 3 sets.
Then go 8 8 7 instead. Really important to not go to failure on your first set.
Best way to improve number of reps is to add in some really slow negatives at the end.
Also make sure you have the right grip, see here
http://www.youtube.com/watch?v=wE3uyUlQoP0&feature=endscreen&NR=1
I was doing this automatically when jumping to grab the pullup bar on the first rep and always was able to do more reps then when I was reaching for something to grab it.