Are you moistening the knot with saliva before tightening
This is a must for me and I use Grand Max or Riverge pretty much exclusively.
There are other dropper knots to consider, one dropper knot I use is a minor variant on the blood knot which I honestly can't explain without actually showing you.... so let's concentrate on the water knot.
It's back to basics I know, but a good guide to the water knot (and others) is on the Greys of Kilsyth fishing knots page:
http://www.graysofkilsyth.com/water-knot.htmAre you joining different thicknesses and/or types of leader material
This can be a pain, but in all cases with the water knot it is imperative you draw the knot tight evenly.
For a wee bit of added security, try adding a half hitch on the fly line end of the knot before nipping off the excess.
H