As the title suggests, I've just replaced the tm lower with a guarda one, prior to this I would have no issues when last round fired the slide would stay open.
Now after replacing it sometimes fails to lock back after last round ! Any ideas ?
I have tried a bit of tape in the mag well but doesn't seem to work.
Wondered if the Spring was weaker for some reason or the super power gas (that I had used before without effect ) may be doing something ?
All parts are standard I've only put a Guarder frame on.
Maybe oiling the spring for the slide catch?
Replace spring for slide catch?
Do you have a 150% recoil spring in it now?
What gas are you using and do you have a fresh can that you can try?
Nuprol stuff is overpriced, but it's alright - I'd try Smart Gas or Pro Gas (they're the same thing). Long-term importing it from Gunfire or Airsoft Entrepot can work out extremely cheap. So do you not have a fresh can? This just sounds like it's short stroking slightly and the slide isn't reaching the slide catch because your green gas is getting empty. You need to try with a new one before proceeding.
Also (and I know you're not stupid but just to be sure) you need to hold the magazines upside down and time how long you're filling them for. The Marui ones don't have silent fill valves so they'll sputter even whilst filling, but will sputter more when they're completely full. If you've not already, time how long it takes until it splutters properly and then fill for a half second less than that - then you won't waste gas. Tends to be about 4 seconds for the 25rd magazines.