Weight lifting can help, but in isolation won't probably make much of a difference. One of my brothers is significantly stronger than I am but cannot hit nearly as hard. The power of my shots come from having better technique and a better kinetic chain.
As others have stated, part of it should be to get good video of your technique and compare with the pros. You may be under-rotating into the shot, rotating too soon, or not timing the parts of the body correctly.
In terms of physical training, you want to strengthen your core (I'd advise doing braces, bird-dogs, and hanging leg raises along with back exercises like deadlift.) You want to train muscles to move explosively using things like throwing medicine balls, and other exercises in which you move explosively. I'm a fan of the speedchain type trainer and of doing some limited training with a racket without strings to increase head speed.
I've found weight training to help me mostly in terms of injury prevention. I've been doing more upper body stuff (pull-ups, deadlifts, rotator cuff stuff and it is allowing me to swing harder with less chance of injury. I believe that my back muscles were comparatively weaker than the chest muscles and that was allowing me to throw my shoulders out.