If it's an old JG...you might want to look at the cutoff lever area. That and how the selector plate is moving when switching modes.
You could manually try to feed some BBs into the hopup...that'd help isolate if it's a mag that's not feeding or some nozzle/hopup issue.
What hopup, nozzle and tappet is in there? After the first shot...the nozzle/tappet/gears/etc...resynch themselves. A mish-mash of parts can cause it to get wacky. A sector gear chip might help...but it's hard to guess at that.
What's the ROF? If it's too fast for the mag...you'll end up with blank shots.
You can go about figuring all that out.....there's lots of stuff written up here and on the Internet about each of these things.
...or you can find a local gun doc and he can just show you what all these things do...