Take them back to the basics of using a hitting tee and soft toss.
Work them on using only the front shoulder/lead arm and have them make their inside turn which I explain to my HS kids as the "lock and load" or weight transfer phase of the swing.
Then work them with only the back, or underneath arm on the same drill.
Videotape them to let them see what the swing is "supposed" to look like.
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