I have been making some parts with hand lay, with EL2 epoxy slow. Now after curring sometimes the parts stay a bit sticky on the surface. I am guessing it has to do something with the temperature while curing the part. I am currently trying to get the room up to temperature while curring, but what can I do about those parts that stay sticky? I don’t prefer to make them again...

You need to get them fairly warm, as now they don't have the heat from the exothermic reaction to help. Mid 40's for a day or so should sort it out, or if you can get it as high as 50 then it should only take a few hours. 

If that's doesn't make it go hard then it isn't a temperature issue and will have been a mixing issue.