Kroil or another good penetrating oil. Let it soak for 6 or 8 hours and try again. Also, try to tighten it first. Sometimes a stuck fastener will break loose if you tighten it first. It's an old trick used on exhaust manifold bolts on automobiles. Try to loosen exhaust manifold bolta without tightening first and they'll likely break. I learned that the hard way being stubborn when I was younger lol
Edit: guess someone beat me to the kroil suggestion lol there's another penetrating fluid I can't recall the name of that works amazingly well. You can watch it actually crawl UP the fastener threads from bottom to top. Trying to remember the company.
