I just got a Mosquito and am still working with it on these issues.
Everyone knows they come with two recoil springs, right? I bought mine used from a guy that did not know there was a second spring in the box. Right next to the manual he did not read, which if he read he might not have sold me the gun. Worked out OK for me.
I switched to the "white" spring and tried Rem and Fed bulk ammo, and the Fed ran 95% while Rem ran 40%. Just not enough umph to make it cycle.
I also tried Winchester, the better stuff with copper plated bullets but not exactly CCI minimags. It ran like a champ all the way.
I have high hopes for the Federal bulk if I can run a full brick thru it. I've given up on running Rem bulk in the Mosquito.