another thing to consider is that most rattlecans have warnings like "recoat within 60 minutes or after 24 hours" or similar. that could also be an issue.
when i paint, i tend to go REALLY fast... i lay down the next color as soon as the first is dry enough that i can coat it in latex. maybe that has something to do with it? maybe i'm getting the colors mixed while it's still curing, and that lets them bond better?