If you're using GW paint, my best guess is that the layer of paint on the surface is just too this overall. Even if you put 20 thin layers, it's still a thick layer of paint and it will crack. My suggestion is to use a "little" bit of astronomican grey foundation paint and skull white as a base (assuming it's a small area) and then use your thin layers of blood red. You use less layers overall, thinning the overall layer of paint. Now, if it's a completely red model, like blood angels marines, then white primer would do a better job.
My 2nd guess only applies to older molds pre-2002 where the amount of wax from the mold was pretty excessive. This can loosen the bonding of your primer.
My last guess is you're just probably handling the model a little too rough from the sides that shifted the layer of paint. Try not touching it at all. After you finished the model, don't forget to add a coat of dullcoat for some form of protection. (others will say gloss coat then dullcoat).
Weather should not play such a large role. I know, I had my models on extreme heats and colds in my car many times.