Marauder bolts ARE stiff to pull back, even when deburred (especially the slot, a lot of friction happens there) and properly lubricated; this is a known condition on a stock model.
When I loaned my Marauder to gunpowder shooters, they inevitably get a double feed until they understand they have to pull way back until the click and all the tension has left the bolt knob.
How to truly fix it is more work than most people want to fuss with. It involves deburring the slot and polishing the part of the air tube the hammer runs through plus lube, also getting a lighter hammer spring (TSS with lightest spring combination), and a valve which has a lighter return spring (such as a Hill valve with optional 7 pound spring), plus adding a regulator (such as HUMA) and dropping the regulated pressure down to about 1700-1900 PSI. Don't forget to port and polish it a bit, with a larger transfer port (like about 0.187 inch for 0.25 caliber), to offset the fact you are using lower pressure. Oh, and all of these things reduce your extreme spread in velocities, increases shot count, and enhances power.
Or,
You can just realize you have to yank back pretty hard to cock the darn thing. This is why many air rifles us a lever to cock, it masks the amount of force you need to apply to cock them.
