Make sure you are getting enough protein. The ideal diet for muscle building contains one gram of protein for every pound of your body weight every day. For most people, this can be achieved through diet alone, and protein supplementation is not usually necessary. Supplementing your daily protein consumption beyond this will usually yield no benefit.
Stretching after you do an exercise can be just as important as stretching during a warm-up! Immediately, after you have worked on a muscle group, do a stretch that focuses on that group and hold the stretch position for a good 90 seconds. This will stretch out the fascia (the connective tissue surrounding the muscle) and encourage more muscle growth. This allows you to have more range of motion at the bottom of every single rep.