Spent a few hours going through all the new G&G rental guns at my local site, and its a problem with the hop unit on all of the new design ones. Simple answer, you need to replace the hop unit completely, the unit is now made of a much softer plastic than they used to use, and the bit that engages the mag gets deformed too easily, and doesn't even engage it with some mags as its too short.
We systematically replaced all the hop units with the metal lonex one along with a lonex hop rubber and sure enough, they have had no problems with any mag type since. One thing though, if you do do this, use the stock g&g barrel clip, yet to find a gun where the lonex one will actually fit in the receiver, courtesy of the pointless reinforcing strips on the outer diameter.