There are a few common mistakes when first learning the swing.
Once someone gets the pattern down...hinge, plank, hinge...no rounded back...they usually then need to spend time working on these two things:
Using legs/hips to drive bell instead of arms. Hips drive, arms guide. I typically have them bump up weight if everything else looks ok and that increase helps.
Timing. These two work together. Using legs will help with timing. At the top you want to let the bell fall and then move the hips out of the way. Instead of bell and hips pushing back at same time. Timing takes practice.
This video is from a Swing Skills Class with Dragonfly members. No one usually leaves 100% perfect, but it's always amazing to see how much improvements happens when you can dedicate time to just one movement. It makes their time in class much more efficient (and fun)!