My buddy runs a GG&G bolt release on his Benelli M4. When he came and took a class from me, he was having problems.
We tracked it to the back side of the front of the extended bolt pad touching the receiver. We took it off and the gun ran flawlessly. Put it back on and he had jams.
I removed material from the back of the pad a bit at a time, using trial and error, until it functioned perfectly.
That's been about a year ago. I called GG&G and talked to one of their engineers and explained the problem to them. Not sure if he just got a bad one, or if it was a flaw in the design. If so, hopefully GG&G has fixed it.