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.
Age and knowledge don't always come together. Sometimes you just get the age.