I hope you didn't replace any of the selector parts with those from the G18C. There's one tiny piece (part no. 253) that isn't cross-compatible. I've got it here as the previous owner of my 23F had the piece from the 26C instead of the correct one. I've already gotten my hands on the correct part but I'd be willing to do a direct swap of the 26C part for your 18C part with you if you need it. See this link for my original posting.
http://www.airsoftcanada.com/showthread.php?t=16343