Its Dulux liquid gloss that I bought last year, used some of it then and it was fine. I wiped down the sanded woodwork with a white spirit- soaked cloth about an hour before painting. And, yes, I did stir the paint thoroughly.
Its likely to be the high humidity thats affecting your paintwork. Here's what is said on the site of a well known paint supplier.
High humidity can interfere with the supply of oxygen necessary for the curing of alkyd paints and can delay the tack-free time from hours to days. With latex paints it is possible for the evaporation of water to be slowed as the paint dries. If the coalescing solvent (film forming glycol) evaporates before the water leaves the film, you will end up with a paint film that is weak or can crack and/or peel.
^^^Maydup...fans don't cool the air - they simply move the warm air about. The air only FEELS cooler to the body because of moisture evaporation from it's surface ( called "cooling by evaporation").
You need a proper (maybe mobile) air-conditioner to cool/dry the air in a room.
When I worked in a pub I used to know some professional painters. If they were working outdoors they used to get laid off if the weather was too hot. That was to do with the drying of the paint . One hot day they told me they had been 'fly'd off' . There was a plague of small flies that were landing in the wet paint and getting stuck. Best excuse for going down the pub I ever heard.