I think what he is saying is that after he has finished shooting he has to release the STI mag to release the slide rather than being able to release the slide with the slide release.
There couldt be a difference in profile between the follower of the STI magazines and the follower of the Norinco magazines. Norinco could have made the bearing surface of the slide release a hair smaller so it would not engage the front left edge of the follower. Your STI magazines should have a generous recess cutout on the left side of the magazine, forward of the lip, which allows the slide release to contact and push down the follower. If this is missing the magazines are defective, but I suspect it is more likely that the Norinco mag release is undersized. An extended mag release is a nice feature, and this upgrade will resolve your problem.