Luftwaffe Fight Tactics (early 1944)

[Pathfinder]

I have read that when attacking B-17 formations, German fighters would normally attack from behind and try to knock out the tail gunner. Later, I believe the tactics changed and the German fighters then began head-on attacks into the front of the Fortress formations. Why would they change tactics to head-on attacks later, as the B-17s were beginning to have chin turrets in early 1944?

Can anyone elaborate on these details and maybe tell me the tactic that might have been employed by German fighters in the April-May 1944 time-frame.

Thank you