Its a draw??

[farquit]

I've been playing HOMM 6 for a few weeks now but all of a sudden some of the battles I enter are recorded as a draw. Because of this I have not been able to defeat 3 of the 7 stacks of Darkness Elementals I'm required to defeat in the last scenario of the Inferno campaign. It means I can't finish this campaign. As soon as I press the Attack button is shows as a draw in the log and the stack is still there. I have also encountered this once in the stronghold campaign when trying to defeat some Vestals to claim an artefact. Any ideas?