I have used it often. I mix equal amounts thoroughly (mix maybe for thirty to forty five seconds), apply the epoxy then walk away for eight hours to let it harden. It has always hardened for me.
I have also hit it with a heat gun to make it run like water and it still hardened in eight hours.

If you have any unused try a test by mixing equal parts of the epoxy and the hardner (or whatever mix is in the instructions) on wax paper and let it set for eight hours. If it sets hard then you mixed that batch well and it means the two items you mix are capable of hardening. If it remains soft then something is wrong with the product, mixing method, environment, etc.

Could it be that you have mixed equal parts of resin or equal parts of hardner? The product usually has different colors for the resin and the hardner. For me one part is a carmel color and the other is clear.

I have used it on a fiberglass/epoxy sailplane fuselage to fill pinholes and fiberglass joints and it sanded like the surrounding epoxy which was really nice.

I also use the five minute version from Devcon and it works fine for me.