marui GBB mags tend to appear to be full when filling. they leak gas during the fill process. this is common and nothing to worry about. depending on the mag, I'll fill until I see liquid propane spewing from the adapter/fill valve. not just the vapors but until I see drops of propane. they vaporize quickly but don't let it spray onto your hands. this can be a 5-7 second fill. some of mine can take more.
make sure the adapter is straight to the fill valve. don't assume the base plate of the mag is 90 degrees from the fill valve. (though some are)