Make sure the charge handle latch is held down /
unlatched.
And you should pull back on it hard also when you slam the butt down.
You can't knock it out with a cleaning rod because the bolt is probably already partially locked.
Only way to unlock it is by getting the bolt carrier to move back until the cam pin unlocks it.
If slamming it fails, pop the pins and take the upper off.
Then you can whack the bolt carrier from the bottom with a wood block & hammer.
I have been most careful to resize my cases properly
Two things can cause this with reloads.
1. Not lubing the inside neck slightly before sizing.
The expander button grabs and pulls the shoulder forward slightly. IE = Headspace is too tight for the bolt to lock.
2. You are crimping the loads.
Unless cases are trimmed to all the same length, a long case will buckle the shoulder imperceptibly when you crimp it. You can't see it, but it will lock up an AR-15 tighter then a bulls patootie.
You can get around it by not crimping, or using a Lee FCD collet die to crimp with. It is not length dependent.
Also, get a Dillon or L.E. Wilson case guage and drop-check your ammo after you reload it.
http://www.lockstock.com/prodinfo.asp?number=WCG223
rc