Gas leaking from the ejection port

[BBrotherwood]

I've been trying to set up my L85 for a game on Sunday. Got my propane bits through the other day so I emptied all gas out of the mags as it was not propane in there. When refilling one of the mags I noticed that it was leaking out of the ejection port at the top of the mag. About 15 minutes later its still hissing out as well. What is the likely cause of this? The other magazines also seem to fill in about 5 seconds, I was under the impression from the size of them it would take quite a bit longer then this?

Cheers