My new JG M4 Commando/ M733 AEG Rifle doesn't load the Hi Cap Mag?

I recently purchased my first air soft gun from Evike.com and I am very new to how everything works. I have fired air soft guns before but I have never owned one. I tried loading my 300 rd Hi Cap mag into the AEG but it doesnt click into place, it fits perfect but it doesnt stay in place. I am almost 100% sure the magazine is compatible with my gun.