An epoxy blush can happen from time to time and you will know it when you see it. Basically, your epoxy finish will look milky white or in cases of the darker colors have a grayish color to the surface.
This can happen because of one of two reasons. Either the epoxy hardener was not mixed with the resin properly or the curing process took place during a time in which the dew point closely matched the outside temperature.
The later scenario usually happens during colder temperatures and as nightfall approaches. You should make sure that your new coating has at least a six hour window of consistent temperature to cure in.
To protect against condensation, you should apply your coating during a time such that condensation will not accumulate on the surface of the coating until it has completely cured. The curing process can also be aided by placing a tarp over the coated area while the epoxy cures.
When mixing, you also need to make sure you scrape the sides of the mix container you are using and mix the epoxy thoroughly. This will ensure that the hardener is mixed into the resin thoroughly. Be sure that you only mix 2 parts of component A with 1 part of component B. A complete kit (quart and half, gallon and half, three gallon) comes pre-measured already.
If you have a case of epoxy blush, do not fret! It is pretty simple to fix. In most cases simply rubbing the area with a rag with lacquer thinner or acetone on it will clear the blemish right up. In worst case scenarios you may have to scuff the area with a green scotch-brite pad to remove the fogginess and then apply new, thin coat over the affected area.