Need help with a FTF issue

[Bigd7467]

I have a S&W Magpul AR with the mid length gas system that has recently developed a failure to feed / double feed issue. I have approx. 750 rounds through the rifle with no memorable malfunctions. While attending a training course the rifle functioned flawlessly on PMC ammo the first day. The second day with the same ammo the rifle began failing to feed, it would extract the spent case most of the time (had only a few double feeds) and either pick up the next round at the back 1/3 causing a partial feed or not feed a round at all. The rifle is also failing to lock back on an empty mag. This leads me to believe that it is short cycling. I tried a different Bolt and Carrier group with very similar results. Only changes between the two days was the addition of Ranger Plates to my P mags and a light coat of grease on the buffer spring ( I cleaned all grease off and the issued continued). The second day was also about 25 degrees cooler( about 45 degrees with light snow). I appreciate all the help I can get.