Have my biceps hit a plateau?

I've been working out my biceps with 15lb dumbbells for about 2 months, standing bicep curls. I can't seem to switch to 20lb dumbbells, they seem way too heavy. I lift about 12 reps every set, i haven't really increased in the 2 months. Should i switch to 10lb dumbbells with more reps to confuse my biceps? Or should i just jump to 20lb dumbbells and try my hardest to lift 6 reps?