I got one for my son when he was little. Not a very good rifle. I had the same problem with his. The problem was the scope was mounted too far back and the hammer was striking the scope. It hit it just enough where the hammer would hit the firing pin, but not fully, firing just part of the time. Check the scope close to the hammer and see if there are any strike marks.
“He grouped his last five shots right in the center of the bull’s-eye. Then I showed him my technique of scattering shots randomly around the target because, as I explained, you never know which way the deer might jump just as you pull the trigger.”