Assuming you've done all the normal stuff (tapping the forward assist, etc) you probably should take it to an expert. Like others have said, you might have a condition where the bolt has snagged on a dislodged piece of the upper receiver, and forcing it could damage part of your rifle.
I'm curious as to how you removed the lower receiver while the bolt was still 1/4 inch back, but given that you managed it, you could try striking the bolt from behind with a plastic or rubber mallet (not steel, to protect the bolt). If you can get it to go fully forward, you may be able to pull back on it from there as a starting pace to work it loose, or get it to unsnag from any debris that it might be caught on.
Kind of a weird problem, but you're not the first person to ever come across an AR doing something weird. Mine once bent a .223 cartridge to almost a 45 degree angle. These things happen. Good luck,-- Tex
8 years using M16/M4 as my service rifle, privately owned AR.