I have replaced hundreds of these, the more than likely causes are:
(1)Glass isn't sitting properly or LCD is fastened down all the way. Ensure that that LCD is screwed down all the way and that the front panel/digitzer is set down all the way - if it isn't you probably have corner or side damage or debris left behind like glass or glue, that is preventing it from sitting down where it needs to be. Fixing damaged corners or sides can be tricky, I use the G-Tool to reshape damaged areas. Also try not to destroy/remove the bezel that is mounted to the frame it is an aid for the glass to sit properly on top of the iPad. You can remove the bezel if needed to reshape, just make sure you put it back down and glue/sticky tape it back to the frame.
(2) Check your connection cable for any slight tears. Check the end of cable where it mounts to the board make sure it free off debris and not damaged. Also make sure that the board is clean where you are mounting the cable/ribbon into, use canned air if needed to blow out any debris. Make sure the cable it set in properly at the frame area - it should have an overlapped/lip bend like an (Z) from glass down to frame and then run down along the side of frame into case to board, it should be secured along the walls/frame. Check the guides here for detailed info. Remove digitizer connection, plug back in and hard reset with holding home and power button at the same time.
(3)Try another front panel, if that doesn't work then it be a board issue. You could try to restore it if you can get it too work, this can fix some but not all.