From sad and bitter experience during the Dick Jauron era, if WRs are continually NOT getting open, it's the HC/OC's fault for not designing better plays. Now, in a particular game, the receivers can have trouble getting open against stout Ds, but if it's happening in game after game, that says the problem is a crappy offensive scheme and not with the receivers themselves.
Obviously, if the defense is putting 6 DBs out on the field, the offense needs to have plays that use more than two receivers and a TE or RB as a checkdown. Otherwise, everybody gets double-covered. Play-not-to-lose-by-too-much coaches do NOT seem to comprehend that. :confused:
OTOH, if WRs are open repeatedly and the QB goes to the double-covered guy or his checkdown, then that's on the QB.