I had a similar problem. I encountered an enchantress, difficulty 8, no other modifies, I rolled 2d6 and got a 6 with a Plus 2, but failed by four. The only thing I can think of, is that twice earlier in the adventure I encountered a surprise attack and failed to defeat it. Could it be possible that the -1 per die wasn't getting removed after the one encounter and it kept building with each failure?
Is it at all possible you were using a character who was not (yet) proficient with weapons, and he/she was using a weapon that adds 4 to the difficulty when wielded by a character who isn't proficient?