I've used both ProBond and RodBond and prefer RodBond, but they are both excellent products and both are very forgiving in terms of amounts mixed. I like Auntie Em delve into the AR type of personality. And due to the very long working time for these paste epoxies I thoroughly mix and measure.
Having said this…that does not seem to have been your problem. It sounds more like a surface prep problem with possible contamination of the surface. I have attached a link to an article about surface prep and epoxy.
Having done a number of repairs/replacements on grips and handles I also find that really glossy blanks that are not prepped properly show a ‘lack’ of good adhesion to the grip or handle. The dried epoxy peels right off the blank.
Hope this helps.
Rodbuilding.org - Surface Preparation