Could be a few things: tight spring holding the things in too well, too much lubricant (or possibly the lack of it) preventing feeding, or making things too slippy to allow feeding, the mag lips being to tight, sh*t BBs (I think we can probably discount that one). Check to make sure there is no raised bit of molding on the feed lip of the mag (if there is, a needle file can sort that out, but be careful and make sure it is the issue before going at it with a file, because you can't put material back lol), have a good look to make sure everything looks like it should work/is aligned properly, it's unlikely to be the case, but not impossible, if the barrel is not lined up or in the gearbox/hop properly, that could cause it not to fire a BB but still to cycle okay.
Can you manually feed a BB into the gun to see if it is actually firing them? Check to ensure there's not one jammed in there, or some other obstruction in the barrel or chamber. New guns quite often have a fair bit of crap on the inside of the barrel as a result of the manufacturing process, so put that cleaning/unjamming to use, you might be surprised how much shite comes out of your 'new' gun, and any appreciable quantity of lubricant which has run down the inside of the barrel could possibly prevent a BB going through. Sometimes that crappy polystyrene the rifle comes packed in can bung things up, it was all over the internals of my SRC AK74U when that arrived because the upper receiver was open in transit, I had to strip it to get all the bits of polystyrene out of the thing, there's really not much clearance between the barrel inside radius and the BB, so even something small can stop stuff going through it..
