Was it an aluminium cap? Those really need to be steel for self-extracting crankarm-bolts.
The arm may have a little anodic corrosion with the spindle. Spray some PB-blaster on the back side to try and loosen it up a little before using the extractor tool. Don't use a coin to block the pusher from going into the BB-spindle, it'll get stuck. Look around for some hardened steel washers at the hardware-store. Mill-washers are thicker and won't bend.