Just throwing it out there but I've read a lot on ASC that VFC lower bodies can sometimes be very particular with certain brands of magazines and proper feeding.
Make sure there is no obstructions with your hop-up chamber as well - such as the hop up sleeve sitting right on the inner barrel and even making sure your hop-up is dialed completely off.
Do you noticed the nozzel moving back and forth while the upper is separated from the lower receiver? The nozzel could appear not to be moving if it is caught up mid cycle when you release the trigger.
In my past experience any feeding issues have almost always been either some obstruction with the hop up or compression/airseal (nozzel, piston, cylinder etc..)
Also this is the post you can refer to:
http://www.airsoftcanada.com/showthread.php?t=126690