I'm afraid I don't know. My experience with the SVT-40 amounts for firing my friends a lot, I've never taken done much detail stripping or maintenance with it. Here's what I do know: The gas system has different settings on it, and some brands of ammo will cycle really well on a certain setting, while they won't do to well on other settings. Many brands of commercial ammo and vintages of surplus prefer somewhat different settings, so you need to experiment a bit until you find a setting that allows your particular brand of ammo to cycle reliably. If you were in the Military in before the late 80's/early 90's, or owned an FN FAL before they were banned, this will be somewhat familiar to you from the old FN's. How you check this, or change the settings is beyond me, however. I'm sorry.
If you've never even taken apart your gas system, this could even be causing this issue for you, as there could be cosmoline and gunk in there preventing it from cycling your rifle properly regardless of gas setting and ammunition used.
Again, I'm not exactly an expert on this subject so if I'm wrong, someone is free to correct me. I'm sure someone who knows their stuff on this particular subject will tell you more.
