There are a couple reasons why the paint will be slow to cure:

1. High humidity (over 30%)

2. Low temperature (below 65 degrees F or 18 degrees C)

2. Heavy coats. 

You can accelerate dry times by blowing cool air from a hairdryer across the paint. Or use a small room fan, about 6 feet away.

Try a painting a swatch on the provided test card indoors where you can control temperature and humidity. It usually takes 3-4 coats to achieve coverage when they are applied properly (thin).