Well that's an easy test since its a double action trigger you can take the mag out and pull the trigger.That's what I thought, but I read that it's something to do with the magazines..
If that doesn't work try pulling the hammer back and trying again. Now do the same with a mag in with no bb and no gas.
Then with gas.
Then with bb's and gas.
Oh and DON'T sand anything until you see if it works without the magazine in!
If it works without the mag its possibly what you read about online, if it doesn't its possibly the trigger spring.