I actually have the same problem with a 14.5 carbine-gassed upper from BCM purchased about 18 months ago. I've been too lazy to send it back in for service. I posted about it here and decided it was maybe me but enough searches indicate it happens on occasion. Here are some other posts here. I know when I talked to BCM (excellent service by the way) they said if I sent it in, they'd take a look at it. That was before Xmas and it got too busy but I'd like to know what happens when you send it back in. I had decided to just buy a Bushmaster 0.040 tall sight but may just send it back.
https://www.m4carbine.net/showthread.php?t=42485
https://www.m4carbine.net/showthread.php?t=54954
BTW, Grant sells the sight posts: http://www.gandrtactical.com/cgi-bin...n&key=TALL-FSP

