Yes the fit of the bit must be exact. A few light taps on the driver while lightly turning the bit while it is in the slot sometimes helps to break them free. Also I've read where the aerosol freeze sprays can help. I don't know f they contain anything that would stain blueing though so I've been reluctant to try it. A ziplock bag of ice over the screw for a few minutes may help loosen a stubborn one.