Is the video (uppermost) plug. I replaced my wife's screen this weekend, same result like you. Reseating the plug ( multiple tries) finally worked. Just make sure you unplug the battery while "having fun". The phone is allergic to hot plugging.

If the top-most plug is not 100% seated (you should hear the snap), than the whole thing is getting "creative". Make sure your battery is disconnected while you play with the plugs: like any computer, the phone can be "allergic" to hot swapping.