My dd's problem may be different than yours, however, my dd gets into a dipping "funk" from time to time also. I've watched for "dropping hands" at beginning of swing, dropping the shoulder by leaning backwards (poor weight balance /posture). After correcting those "basics" through RVP with her hitting coach though, she was still swinging "WAY" beneath the ball sometimes and it was not her hands at swing initiation nor weight distribution problems, ... it was her front left foot.
She was opening her toe on her left foot during her step/stride, which allowed her knee to bend slightly forward towards the pitcher during weight shift rather than having a locked front leg to swing "against", which in turn allowed her entire body to sink 2-4 inches occasionally. So, she was either swinging way underneath the ball, or popping up when she adjusted other things to at least make contact. One of her "swing" keys now is to make sure that her left (front) striding foot remains closed, sometimes exaggerating it during on-deck prep.
If it is a weight distribution issue (leaning back), there is a drill that Glenn McCoy used with dd for making sure her weight is moving forward during her swing - good for power as well as eliminating a leaning backwards & dipping the back shoulder problem. It is a step over drill where you hit a ball off a tee while walking towards the tee, with a crossover step just prior to swinging. Not sure how to explain it better than that, but it gets you in the feeling & habit of moving forward aggressively into the ball and getting your weight shifting through he ball during your swing. Hope I didn't butcher that concept too bad Glenn / hitting instructors.