if your move cost for diagonal is the same for lateral then it wont know which to do, usually if your sideways move cost is say 1 then your diagonal should be 1.4 (because sqrt(2) is 1.4) not sure if this is your problem i could be wrong!
I also made a larger, slower enemy that shoots 2 bullets at a time and when it is killed a spawns one of the smaller green one! but I can't think of any other types of enemies :/