Check that; if you have the 8mm self-extracting bolt and it was installed correctly with the specified amount of torque, it takes quite a bit of effort to get that sucker back out. I tend to use the same torque wrench / breaker bar to remove those self-extracting bolts as the one I use to install and torque them to 390-485 in.lbs as specified.
http://darelldd.com/bicycle/document...r%20cranks.pdf

I started off my original reply based on the FSA Mega EXO crank design on our triplet's captain's crank... which used a pinch bolt and preload screw cap. Sorry for any mis-direction.