If the resin is hard but the surface is sticky then you need wax (or more of it) if the it just stays rubbery in the pot you didn't put enough hardener in it...

Here is the quick and dirty assessment, lot of problems are caused by not mixing it well...too many people put the MEKP in the resin and run the stir stick around the container a couple times....you need to thoroughly mix it...also you don't work out of you mixing cup...especially with small batches, as the resin on the sides of the cup rarely get completely mixed...so mix in one cup then pour it into another to actually work from...those two things will save you a lot of headaches.

Did you stir the resin can well before you poured off what you were going to use? If not, the wax will not be mixed throughout the resin and if you do little batches at a time, by the time you are half way thru the can, most of the wax has been used (floats to the top) and your gelcoat surface won't harden . I have found that most Gelcoats that come "with wax" seem to have a marginal amount of wax at best, I make it a practice to put in an additional small amount to insure a good hard sandable surface.

In general it sounds like you went light on the MEKP but its hard to say for sure as they there are other factors to consider...

Regards,
Bob