RWH: JB Mauser asked you all the right questions and one of the other fellows hit on yet another possible problem with either reloads using the wrong powder or possibly some of the light magnum ammo not using fast enough buring powder to cycle the action.
I would first run through JBs list, then check your ammo. If you are purchasing mil-surp ammo, make certain it is American made. Not all foreign made mil-surp is up to American standards.
Merry Christmas. Mikey.