At some point, your high-cap MP5 airsoft magazine may give you some trouble. It's usually not hard to fix. You just have to disassemble it and fix the problem. This article shows you how to take apart the MP5 high-cap magazine.
To disassemble the MP5 airsoft high-cap magazine, you have to tap out the pin at the top of the magazine (see photo below). To tap it out, you'll need a punch with a rubber mallet.
Once the pin has been tapped out, you can slide the magazine insert out of the metal case. The photo below shows the magazine insert.
Flip the insert over and you'll see three screws (see photo below). Remove these three screws to take the insert apart. Be careful that you don't loose the spring at the mouth of the BB feeder.
See anything wrong in the photo below?
The locking washer is gone. It has gotten stuck inside the insert where the BB feeds. This is one of the most common failure mode of the high-cap MP5 airsoft magazine. Once you retrieve the locking washer out of the gears, put it back in place and your magazine should work like new.