I have run into this problem from time to time.
Here is what's happening: the mag hole is punched a bit too low, so the mag is not catching high enough in the magwell. It's not off by much but just enough so BB's won't feed into the hop-up chamber. as you said if you hold the mag base up it will feed fine.
What you need to do is grind the bottom edge down a little bit (usually less than 1mm), this will allow the mag to seat further up the magwell, and thus should feed properly.
Apply strips of tape as needed to alleviate any wobble, and to help the mag stay seated up in the magwell.