If she doesn't respond to the leg, you need to take a step back in your training and go back to basics, to establish the give to the leg aids.
Try asking for a couple of steps of leg yield here and there, on a circle, out of a circle to a straight line etc. Only a few steps at a time, not a full diagonal or half arena.
Feel like you are 'juggling' her between your legs, put your left leg on and expect her to go to the right and fill up your right hand, and vice versa.
Once you get good lateral control of her while asking for only a few steps at a time, then start looking at working mroe solidly on your leg yield. I find that generally when the quarters are trailing in any lateral movement, it is because you have no got enough connection between the inside leg to the outside rein, and have then got no ability to give a half halt to ask the quarters over.
Try riding 2 steps of leg yield, then straight, then 2 steps of leg yield, straight, 4 steps of leg yield, straight.... so on.
Riding a few straight steps will bring the quarters back in line and give you control of the shoulder again.
Same process for tavers, you need to control the shoulders and have a very good connection from leg to hand. Riding dressage is very much a juggling act. You can never rest and 'celebrate', we're always asking for a little more flexion, a little more energy, a little more over the back....