Quoted: Is the Bolt completely forward???  Try hitting the forward assist.  My buddy took his AR apart then put it back together and for some reason unknown his got stuck and we couldn't get it loose.  I hit the forward assist and then everything released.  Drop the mag and try that.  If that doesn't work seperate the upper and lower and have someone pull on the latch side of the CH while you hit the other side with a rubber mallet or a block of wood and hammer.  The CH can take a good amount of abuse.  GL

DON'T hammer on the charging handle.
The proper clearance drill for this is to buttstroke the rifle while applying constant pressure to the charging handle.
Let the mass of the bolt carrier act as a puller to pull the cartridge out of the chamber.

Buttstroke drill:
Holding the rifle as if you are pulling the charging handle back, stroke the whole rifle down against the ground so the butt of the stock hits the solid ground.
Maintain pressure on the charging handle during this.
One or two good hits should do it.
If you have a collapsable stock, you will want to fully collapse it, or better yet, remove the stock and just stroke against the buffer tube.