No clue on filing the bolt catch.
Sounds like you've got some dirt in your knocker valve. If you have a valve key unscrew the knocker valve from the mag and inspect/clean it. I do not know the name for it, but on the knocker valve itself there is a piece of metal (shaped like a c) that holds down a cup containing the spring which closes the valve. If you carefully remove this piece of metal the valve comes apart and you can fully clean and inspect the valve.
And when I say careful I mean it! The little C piece can go flying if you're not (good luck finding it if it launches itself across the room), same goes for the metal cup and the spring it holds although it won't fly as far... Learned this the hard way
If you don't have a valve key, go get one. They're not too costly and you're going to need it to be able to properly maintain your gas mags. If you have a GBB pistol it will also help you there if you need to work on any of the seals on those types of mags too.
After you remove the valve, disassemble it and clean all the surfaces (use q-tips or tightly rolled up paper towel to clean out the hole that the valve stem passes through. Inspect the o-rings for any damage, if they've got any tears or bits of rubber missing you might need to replace them. After you've cleaned the thing thoroughly reassemble it and lube up all the o-rings with grenade oil. Let it soak in for a bit (I used to just let them sit in a shot glass full of oil for a couple hours), shake off some of the excess oil and then install the valve back into the mag again. Make sure to screw it in nice and tight, nothing crazy though.
Hope that helps!