If you're getting stuck at a weight, force yourself to go higher, you might only be able to do 3 or 4 reps at a time, but if you keep going back, pausing for a minute or so in-between, you'll force your muscles to work and you'll be able to do more and more reps.
Doing 10-20 reps of 30 is great if you want to tone up. If you want to build on strength/bulk up, you have to push your body and force the muscles to their limits. You're actually tearing them, that is what builds stronger muscles.