Take a look at the trigger mechanism and the part that holds the piston back it may be a broken spring in the trigger
If not could be a mainspring coil broken stopping it cocking
The only Beeman Sportsman Rifle I can find takes either .177 or .22 caliber pellets depending on which barrel is fitted.
If the pellet you tried was a .177 try a .22.
Otherwise try emailing [email protected] and see what they say.