Can Only Hit Mobs 1/2 Of The Time

When I click on mobs/monsters to hit them (ex: cows, sheep, pigs, zombies, creepers, etc.) it only hits them half the time. This problem doesn't occur with blocks, only 'living things'. This makes it very difficult to defend myself as if I hit a zombie five times, it'll only get hit issues but haven't come across this one yet. Did anybody else have this problem?