Epoxy paint manufacturers all recommend that the Part A and Part B be mixed separately with different stirring devices before measuring and mixing. That is because some components in the bottle are heavier than others, and may tend to settle out or stratify over time. This is much more of a problem with paint than with unfilled resin due to the pigments, but I suppose it could also happen with clear resin. There are other possible answers, but if you are certain about measuring and mixing most of them are eliminated. I always give each bottle a swirl and a shake before measuring.