I'm not 100% certain but I believe the ICS sig has a proprietary nozzle, I think it's slightly longer. If it's been replaced that could be the issue.
Edit* I know you were talking about Marui mags but this is how I got my mag brand mags working nicely in my ICS sig if it helps.
http://www.airsoftcanada.com/showthread.php?t=117937