Pretty much what Brian said.
Do you know the max COAL that will fit in your mag? Do you have a way of measuring how far out the lands are in your rifle. (this is very easy with the bolt, pump, and lever actions that I reload, but I have no experience with ARs)
I've had very good luck with Barnes bullets loading them to the recommended starting distance from the lands that Barnes suggests (confirm this on barnes website before you try it, but I think that measurement is 0.05" from the lands)
Also if you are loading your rounds much longer than the recommended COAL in your load manual than you will experience some drop in velocity with the same powder charge. (I find it significant in large diameter cases such as the 300 win mag when loading "long for caliber bullets", it may not be as noticeable in a small round like the one you are assembling) But if you are consistently comming up short on velocity compared to your book loads, that is probably part of your reason. Along with the typical gun to gun varianace of course.