I'd start by removing the bolt and charging handle. Push the buffer back with a finger and see what kind of resitance you get from the buffer spring. If that's good, start with just putting the handle back in the gun without the bolt, move it back and forth to see what kind of resitance you get if any. And last take out the handle and put the bolt back in the receiver without the charging handle. It should slide back and forth in the receiver, even without lube it should move freely.
Scott

