M4 Mag doesnt work

Ok, so I put the bbs in and they wind, half way though!
The spring also feels very tight, like its already wound...
HELP!

You ARE putting the BBs in the trap door, not in the hole, right?

Why yes, I'm not stupid, well maybe a little

Hahah okay good. Try pressing the release tab (the little button/switch next to the hole that feeds the BBs) and then empty the entire reservoir of BBs.. Then shake it and see if you can hear rattling. If you can, try to shake those BBs out from the trap door and if that doesn't work, hold the mag upside down and see if the BB(s) show at the feeding hole. If they do, press the release tab to make them fall out. Shake it again. If you hear rattling then repeat the emptying and upside down thing until you don't hear any rattling. If you still can't get the rattling to go away then something might be stuck. If you don't hear anything, reload the magazine about half-capacity and see if it works.

i had this problem, the feeding wheel is stuck you need to upen the mag up and get the bb's out but first try what drummerboy said and see if it works.