First step would be to check that the screen cables are properly seated. Be careful here, because too much pressure can damage the connector on the logic board if it is not properly aligned. The connectors should click into place, and the EMI shield will keep them secured.
If you have a good connection and the problem persists, you most likely have a bad part or a damaged cable. It is very easy to tear it when feeding it through the housing, so be sure that the wide, folded portion of the digitizer cable is completely through the slot before securing the screen. If it does not easily reach the connectors on the logic board, '''DO NOT PULL ON IT!''' Pulling on a pinched cable will almost certainly tear it, and you'll have to start all over with a new part.