It is the new damage model. I've had it happen to me a few times. I enage a plane that seems to have no damage shoot it down and nothing.
On the flip side I've hit faster planes that manage to extend away. I give up the chase and go find another fight 5 -10 minutes later I get a kill messege. No doubt some other guy just polished off my previous adversary and is scratching his head wondering where his kill or asst messege is. I guess you could say it all evens out.
if this is the actual cause then its not the new damage model........ its the new damage model bug
the new damage model is supposed to be if you put significant enough damage to a plane (entire wing off, entire tail section off, flaming, etc) then no ADDITIONAL damage can be taken........
this does not necessarily mean the person who did the "significant" damage will be awarded the kill......... its possible that wing might have only needed 1 more round to take it off..... in which case the person who did the prior damage would get the kill but the person who took the wing off would still get an assist
if you encounter a plane that has not suffered "significant" damage such as those listed above......... then at the very least you should be awarded an assist if you even put a single round into him
if its functioning otherwise then it is a bug that is not part of the intended flight model