I probably shouldn't have to ask, but did you rinse the miniatures off after their extended soap bath?
Also; what kind of plastic was used on the miniatures? I hear some types of resin, because being ever so slightly porous, can soak up moisture to some degree. If this happened with your soap, it might actually prevent the paint from bonding with the plastic.
My experience is that just a short wash with some dish soap and a rinse under the tap will work just fine; no need to submerge them for 3 days.
And of course cold and moisture in the air can really fluff up any spraying you do. I've had this happening myself a number of times as well. Try warming up your spray cans before applying the paint. I put them on a radiator for a couple of minutes during cold spells, but I've seen people using a bucket of warm water for this as well.
Moisture in the air is harder to avoid though. It's most prevalent when it's cold, but tropical climates can be just as moist obviously. I've found no real solution for that other than to wait it out or spray inside.
But you'd better be ready for some incoming flak; my significant other will throw a fit when she smells too much paint fumes in the house...
