Yup, it's hard to hit them sometimes... and in general i became dissapointed with the game. It's not as good as i was expecting (and also the game loading system sucks... at least using the original one... ). For hitting the enemies on your back you cal either use the sticks and hit them (sometimes is hard) or use one of the combo moves like the double low kick... it's usually the best way.
Anyways... the game is not as good as i expected.... one of those games i should have not bought if played before....
Not worthy for buying... maybe worthy for copying and playing a couple days, but not a lot more.