Heyyo, folks.

Our GSG 5 has been having this failure perhaps every 5 rounds with several different magazines (both OEM German made and promag) in this way.

I will be shooting, then it will jam and I will open the gun up, and the struck round will remain in the chamber, with the extractor failing to extract it. We have used a variety of ammunition, including CCI MiniMags. Can you recommend anything to try?

My guess is the barrel has moved a little.  The barrels are threaded into the trunion block.  The barrel (especially if its well lubed with oil) will sometimes rotate forward.  You can unscrew the faux suppressor, and tighten the barrel back up.  If the barrel moves forward even slightly like this, it will not strike reliably or eject - similar to what you describe.

I had the same problem, I took mine apart, de greased the threads and used loctite - problem disappeared completely.

You may just be able to tighten the barrel from the front rotating it without disassembly but if oil is in there, it may happen again over time.  

If your saying the round never fired it certainly would not eject because the bolt carrier did not move to the rear.

While true, I would assume that he means won't eject the live /unfired round when manually engaging , (Pulling back on the cocking handle) I think this is because if the barrel has moved forward the extractor won't get a good fit around the rim of the cartridge, it and the bolt are stopped by the trunnion.