your muscles will either grow from progresively more weight at a rep range of 6 to 10 or getting more reps of a same weight.
If I was you I would look at your diet make sure that is right and there would be no reason why you wont grow all over.
A good routine for bi's and tri's would be:
Standing ez bar curls 3 sets 8,8,8
Incline dumbell curls 3 sets 8,8,8
one arm preacher curls 3 sets 8,8,8
Close grip bench press with ez bar 3 sets 8,8,8
French curls 3 sets 8.8.8
One arm overhead dumbell extensions 3 sets 8.8.8
If you do bi's and tri's together then superset a tricep exercise with a bicep exercise.