It appears that since the last two updates a good portion of the zombies are not registering bullet hits. Yesterday I was killed in a guard house because only the first 2 or 3 zombies took damage from bullets, the remaining 5 or 6 that clogged the exit and surrounded me were unaffected by guns. They do still seem to take melee damage. This hit registration is a critical flaw with the increased zombie presence and their increased ability to aggro other zombies.