Originally Posted by spclark

You using carbide drill bits? Those're meant for use on harder stuff, but they break easier (being rather brittle), work better at slower speeds with lubrication between drill tip and item being drilled out.

Too, sometimes it's worth trying to make a stuck bolt even tighter first; turn it to the right before going back counter-clockwise to loosen. 'Course this is opposite to what you want to try if it's a LEFT HAND threaded fastener! (You confident those bolts are right-hand threaded?)