Unable to use Berserker? Need Help.

[XxBlueSpade2xX]

I'm on the last level. After you beat the two snake/worm things that come out of the ground and then you go through the right gate. 2 big enemies and 1 medium enemy come out. My LB is flashing and the first time I tried it I went into Berserker no problem. Now, every time I reload I can't go into Berserker. I really don't want to have to grind out this one part without it. I've reloaded the game twice and nothing. Anyone else having this problem?

Help please :).

Thanks guys, but I got it.

EDIT: Just in case anyone is having the same problem. I reloaded the my saved game another 2 times before I could get Berserker working.