Lets say the Germans are able to add several good divisions (4 infantry, 2 panzer) to Normandy around July 1 1944 due to one or more of:
a) Germany uses more divisions from 15th, 1st, and 19th army areas earlier, held back OTL.
b) Germans correctly anticipate Soviet intentions for Bagration and the Soviet offensive is less successful. More reserves are available.
c) Germans evacuate 17th army earlier from Crimea creating reserves just now ready after refitting.
d) More rational defensive policy in the east September 42-June 44 just has more reserves available that can be sent east to west.
The front isn't that long and there is good defensive terrain. Can the Germans create a density of defense that is just hard to break through?