Hypertrophy is just an adaptive response to a particular kind of stress to the muscle cell. When you train in an 8-12 rep range (with your last reps being difficult) you are in the optimal range (for whatever muscle you are working) to achieve hypertrophy. Along with that rep scheme, growth occurs from the negative (or eccentric) portion of the motion. When you contract your muscle (like in a curl, when you raise the weight), you have uniform contraction of all the muscles involved. When you are lowering the weight, you are both lengthening and contracting simultaneously (or non-uniform contraction), which threatens the structural integrity of the cell. Your body's response is to try to repair that microscopic damage, which is where the actual growth part takes place.
Try working your full body when you train. Pick 5 or 6 exercises that hit every major group in your body (avoiding isolating exercises or machines), use the formula that I described above, and you will grow.