Broken Airsoft gun. any idea whats broken and or how to fix it?

I bought a DPMS panther arms "panther carbine" ( a M16 replica) and it stop working after 3 days of use. I already opened up the gun and there is no jam. the moter spins when i take it out of the gun, but not in the gun. i would fire 10-15 bbs before it would stop working again the I word have to rotate the head of the moter and now it just stopped.