Generally, when lifters ask about something like that, they're more specific about it - for example, inquiring about certain exercises, set-rep patterns and pre- and post workout meals. You need to consider several factors when you want to bring up a weak muscle group and build new muscle in general. So with that in mind let's get to a top-10 list:
Most important rules
1) Train the weak muscle group two times per weak, one day with full and one day with an abbreviated workout of perhaps half normal volume. Make sure to take at least two days of rest between the workouts.
2) At least one day per week train the weak area first in your workout so you can prioritize your effort. You need to hit a weak area when you're mentally and physically fresh.
3) Accentuate the eccentric contraction on all exercises for the body part. I suggest something like a 4/1/X tempo, in which you lower the weight over four seconds, pause for one second at the midpoint and then explode during the concentric contraction.
4) Get extra sleep both the night before training a weak body part and the night after you train it. Extra rest means extra repair time, which equals more efficient growth.
5) Vary your training protocol. You can't hit a weak muscle group with the same type of workout over and over - that simply does not work. Make sure to vary rep ranges, exercises, grip, angles, rest between sets and intensity techniques.
6) During your workout for a lagging body part, sip on a drink that contains 10 grams each of glutamine and BCAAs, along with three grams of creatine and beta-alanine.
7) Increase overall calories from quality foods on the day you train a stubborn muscle. Take in a bit more carbohydrate in the meal that precedes the workout and the one that follows it. You need extra nutrients to train harder and recover more efficiently.
8) If you don't normally train with a partner, try it when training a body part you're trying to improve. Not only can a good partner provide extra verbal motivation, but he or she can also help you take some of your sets beyond failure and can spot you so you can more safely attempt to lift heavier weights than usual.
9) Use therapeutic modalities like deep-tissue massage for a lagging muscle. That can help increase blood and nutrient flow to muscle, break up adhesions and open up the fascia. It will all lead to greater recuperation and increase the muscle's potential for hypertrophy.
10) Use visualization to create a more powerful mind/muscle connection. When working a nonresponsive muscle group, try to forge a very intense link with it during every rep of every set. Simply lifting a weight from point A to point B may not be enough to stimulate a stubborn muscle into growth. You need to visualize exactly what you want the muscle to look like and how you want the muscle to fell while training.
If I had to name a number-one priority, it would be to stay patient. Only with consistency, hard work, an intelligent plan and the patience to let it all take effect will you see the improvements you're after.