I've bought 6 S. Arms M4 mags from AEX SD over the past few months and until now they've worked great and have had flawless feeding. My problem now is that in preparation for DD's game this Saturday I've been cleaning/testing all of my guns and gear. When I went to test my gun I found that I was having feeding problems with the mag that is usually the first/main one that I use. It would feed a single BB then none at all unless I pulled it out of the gun slapped it against my thigh and reloaded it or flipped the gun upside down.

To find out what was jamming it I let out one BB and then looked inside it. I saw 2 BBs, one on top of and slightly to the side of the other (similar to how a double stacked GBB pistol mag stacks BBs). I decided to test the other S Arms mags I had and the other 5 worked. The 5 working ones however, did the exact same thing when I went to unload them later after confirming they worked.

Does anyone know how to fix this and why they're jamming so easily and so soon after I bought them? I haven't abused them much at all aside from dropping them in my dump pouch during games.

Several of the mid-caps I have owned have malfunctioned. Some fed for a while, some never fed. A solution to not feeding is often to take a file, and grind a little bit of plastic away from the feed lip catch (the little spring-loaded piece that holds the bbs in place. File it down (obviously too much filing will ruin the mag) and it should feed much better. Upon re-reading your post, it sounds like you have jamming issues farther down in the mag. I usually just smack the mag, but another solution would be to spray a shot of silicone into the mag. This is, however a double edged sword because that oil can attach itself to the bb which will transfer it to your hop-up bucking. This will not harm it, but as long as there is oil on the bucking your hop up will suffer. You can either clean the bucking or fire a few hundred rounds through. Also, the more oil you have in your mag, the more dirt you will collect. And if your mag gets dirty, that is pretty much the end unless you want to go through a lot of trouble cleaning it.

You could take apart the magazine. The spring may have lost some of its tension and loading the magazine to the max compresses the spring. Some of my MAG magazines have this issue, so I've actually stretched the springs out a little, and lightly lubed the spring before putting the magazine back together. Also it helps not to load the magazines the night before a game so the springs don't stay compressed for too long.

Didn't you recently mess with your hopup? Check your hop to make sure its not what is causing your issue. Make sure that the screws are all the way tight on your lower hopup piece, or it won't feed well.

My other 2 non-S Arms midcaps and both of my VN hi-caps feed flawlessly.

update:
I took the problem mag apart, pulled on the spring a bit and put it back in. I don't think I even stretched it much (if at all) but when I put it back together it worked just fine loading/unloading with a loading tube. I went and test fired 50 or so rounds and it worked just fine so something I did worked.