Digitizer not working after dropped and not cracked

I run a repair business and I receive ipads from a local school district. I usually get 10 ipads. Most need new LCDS but out of the 10 usually 2 have a non responsive touch screen although it is in a otterbox case, its almost never cracked. I've even replaced the digitizers and they dont fix the issue. They also do something weird where they will only charge to about 2% then just give a reboot cycle. I am wondering what IC would cause this problem. I have replaced many ipads and this is very rare but never know what is the cause and just end up using them for parts.