Carbines are very ammunition sensitive. If you are using reloads, the cartridge must be exactly the correct OAL. If they are too long, the tip of the bullet will drag against the front wall of the magazine and not feed well. 30 rd magazines are always trouble, the are just too heavy and if the nibs are worn on the back, they will rattle all around in there.
As far as not firing, the recoil spring is usually the culprit and the carbine should be greased just like the big rifle. Before doing anything, I'd make sure I had good factory ammunition and new 15 rd magazines. Start and go from there. The last time I fired my NPM, it was giving me trouble and it turned out to be debris, dirt, carbon general guck under the extractor. I disassembled the bolt and cleaned it and problems disappeared.
Don't lose hope, these guns are very,very good firearms. The fact that they have been used for 76 years is good advertisement.