Perhaps the person who installed it tightened the crap out of it, which you are not supposed to. Either that or no lube was used on the threads when it was installed.
My advise is to put a few drops of an oil like tri flow into the gap in the crank arm, aiming for the cap. I believe the cap on the DA cranks is alloy instead of plastic like the rest of them so some lube may help free the cap. If you have a rubber mallet try lightly tapping it a bit in hopes that the oil penetrates into the threads a bit.
Good luck.