I have always found the gravity top feed units less troublesome . The bottom syphon feed often foul due to moisture in the medium if temp swings occur during damp seasons that often affect the moisture content of the blasting medium in use and while in storage . I found that more cleaning of the gun nozzle and siphon line was needed and storing the blasting medium in dry environment helped considerably . Adding a vibrator to the medium storage bin will agitate and reduce clumping of the sand . Some units use a air dyer to ensure dry medium . Last but not least medium sand grit sizing is important also to reduce clogging and should be sized according to the manufacturers recommendation for the nozzle design of the unit . Hope this helps Bud !