Humidity can cause frosting, but improper mixing is another common culprit. Extra shaking can never hurt (well, arms, perhaps, but never the quality of the finish), but it's best to avoid spraying in high-humidity environments, as well.
As for correcting your current issue, a coat of gloss varnish may be able to erase the haze, after which you can apply a new coat of matte (with greater care, I hope

) to restore the desired flat finish.