Enduro-Var sprays beautifully if the gun is absolutely clean and does not have any other finish residue. Proper cleaning with acetone followed with hot water should purge the fluid passage. Enduro-Var itself will not cause gun failure.
Another possibility is the power of the gun. Some of the lower quality guns can overheat causing the product to cure in the gun - they need a rest once in a while if you are using them all day.
Also note that Enduro-Var ADHERES WELL only over WATER STAINS, DYE STAINS, and RAW WOOD.
      
  Enduro Water-Based Conversion Varnish
  Enduro Water Based Stain Blocker Primer
  Enduro Water-Based Tintable 2K White Poly
  Enduro Water-Based Pro Series White Polyurethane
  Hard Wax Oil & Hardener
  Gel Stains
  Pro Image Flooring Topcoat
  Jen Poly Brushes
  Stir Stix
  Ultraflex Softback Sanding Sponge