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.