This article is from the Fitness FAQ, by Jeff Gleixner (firstname.lastname@example.org) with numerous contributions by others.
There are basically four things to look at. First, you may
be over training. Try taking a week off and when you come back
take it easy for a few weeks and evaluate your work out. Second,
eat, eat, eat! You'll never get bigger if you don't give your
body the resources to rebuild itself. Make sure the food you're
eating is nutritious. Third, your body may be used to doing the
same thing every work out. Try changing your exercises. Finally,
you may be hitting your genetic peak. Not everyone can have 20"
arms. Basically, taking a break, eating more and changing your
work out should help when you hit a growth plateau.