Did you solve your problem?
Does the mag have a little play or is it nice and snug [I've seen several with loose mags] is it an SVT-40 or SVT-38 mag?
Mine was [SVT-38] pinned short and gave me all kinds of similar problems.
When you load the mag is there any room left? can you push down on the top round and still get it to move 3-5mm?
if not: DON't do this and DON't tell the RCMP but:
drill out the pin, remove the follower, grind out a notch a couple mm deep where it would be contacting the pin - reassemble and see if you can push the top round down a mm or two, there should be a little play there. if not grind a little more - baby steps, if you over do it you will have a big mess on your hands.
when done PUT THE PIN BACK IN! unless you want to go to hell.
make sure you can do all this in one sitting, having an unpinned mag is a criminal offence, they will take all your guns forever if caught.
[oh yea, BTW, lubrication

that's why they were issued with oilers]