My blaster only lets me single-fire if I hold it upside-down.

Hi, I have a problem with my Strongarm, if I hold it in a normal position, single-fire DOES NOT work, the blaster will automatically slam-fire. I had a thought that the single/slam-fire mechanism might be jammed on slam-fire, until I turned the blaster upside-down. The blaster only single-fires if the blaster is held upside-down. I am unsure how to fix it.