Are you filling it properly? As in the liquid propane is actually going into the magazine and not spraying everywhere?
It takes a few times to get the hang of it.
Follow this video: YouTube - Filling a Gas blow back mag
The first fill is good the the second one you can see it's not so good since the liquid was turning into gas and spraying everywhere.
Also NEVER press the "big button" on the top of the mag while filling it. That's the "gas dump" button and you shouldn't really ever press it if the mag is full or the seals can get frozen and worn and all that which means you'll have to either repair the mag or buy a new one.