Is there something wrong with my JG M4A1 airsoft gun?

ok i just got a Jg M4A1 its sick nasty. but heres the thing. if i want to shoot i have to fill the clip with about 30 (give or take) bbs. the clip holds 300. so i can add more than 30. then turn the gear wind up on the clip, then i can shoot fine, but it seems when i have about again 30 (give or take) bbs left in the mag they wont shoot, i try to crank it but the wheel doesnt tighten like its suppose to. i tried shooting the gun upside down with the 30 or so bbs still in the mag and they shoot (but the gun has to be upside down) its really wierd i mean its not a huge gigantic problem but i dont wanna reload my clip unless its dry (out of ammo) afterall i waste those valuable shots. so if some 1 could please help me and tell me how to fix this or if my gun or clip is defected that would be very extremely helpful (will choose best answer) Thx!!