I have had this problem in the past, the problem was not the way I mixed the polyester but the fact that I had some epoxy (cured) on the surface that I glassed. Every spot where there was epoxy the polyester resin did not cure. This was rather confusing because I am sure that I had been able to glass with polyester over epoxy before this.
I guess I had just chosen two resins that were VERY incompatible.