Anything time you dive under the bar leading with the chest, you will always be out of position to receive the bar. While doing snatches you may even hit yourself in the head. One of the problems may be that you are teaching your athletes to pull the bar up to their shoulders or overhead instead of extending the body fast with long arms and then pull under the bar during both disciplines. The reason they don't want to drop under the bar is they are muscling the weight and they lose momentum on the barbell. Drop snatches/Snatch Balances would be a good exercise for the snatch, but also snatches from the hang position with long arms would be another good to teach them to get under. just extend the legs with the torso straight and pull under the bar. The same thing for the clean. Make sure the elbows are out so they keep the bar close. Make sure they are using light weights, Lots of reps!!
Sorry about the brief discription of this, I am limited for time.
