Are you saying it jams feeding or the round chambers and will not fire? Sounds like the latter to me. If so look at the firing pin indent on the rim and see if it looks "normal". Could be the CCI has a softer primer, or the Ruger has a lighter fireing pin spring. Dirty chamber maybe? CCI makes my favorite rimfire ammo. And the budget milk cartons or bulk ammo are my last pick. Usually get bent rounds and whatnot.
P.S. Southern Utah, Marlin 39 Cent is Awesome, Thanks.