Sorry, I meant you must aim downwards to hit *crawling* enemies.

Hi, this is not a bug. You need to aim downwards to hit zombies. I believe while aiming, downwards is default mapped to the S key. You can reconfigure all of your controls in the Options -> Controls menu to make this easier as well.

You can also aim upwards as well to hit standing enemies (except for the reptile enemies) and do bonus damage. Aiming upwards and firing Shotgun close range results in maximum damage and knockback.