When you installed the rail, you could have gotten the gas block slightly off line. The op-rod/slide part could be rubbing the underside of the rail due to this. That would slow down the ejection stroke enough to cause this. It has happend to quite a few persons who install the scout rails. Check to see if any contact has been made there.
This thread details the experience.
http://www.perfectunion.com/vb/showthread.php?t=73707