The only reason for it that I can see is blushing. I'm not sure if the XCR is supposed to be non-blushing, I imagine it would be. But in most cases the only reason an epoxy won't bond to itself is if it develops amine blush on the surface and the salts become a contaminant that prevents a good bond. 

Iv had the same issues number of times,  keep getting told it's mixing ratios,  which I can't see how it can be as I use digital scales.

I've been refurbishing a kayak paddle (blades) by sanding back and applying coats of XCR laminating resin. All has been working well until I tried to apply multiple coats without the full 24 cure in-between.