It could be galvanic corrosion that caused the parts of two different metals to seize. Even greased parts can fuse together from galvanic action given the right conditions. As others have stated, I often use anti-seize compound when assembling two different metals but also use high quality grease if it's handy. More importantly I also move or remove them every couple of weeks so grease is sufficient. Your mileage may vary depending if you live near the coast or in a wet humid environment.

Stop what you're doing and get a can of Kroil. A good industrial supply company should have it. My local Ace Hardware has it as a matter of fact. Douse a generous amount of Kroil on the remaining parts and wait a couple of hours then try again. If still stuck try lightly tapping with a small hammer and brass rod to loosen the "stuckage" without denting the metal. A paper towel between the part and the brass rod helps too.

Good luck!

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