I tested out a variety of different changes to royal icing for making ri transfers, one particular batch never dried, was sticky like damp taffy. Is far as I was able to determine, it was because a tiny smidge of vegetable shortening was on either the spatula or bowl. I was not aware of grease getting into it, but it was the only explination I came up with.
I used the same icing for a piping ri transfers in a lighter and a darker version, and one of a thinner consistency. Next I paused and did something else that involved very lightly greasing a pan with shortening (crisco). I then thickened the icing with a little extra sugar to thicken it, mixed well, and piped. It appeared the same while wet, but did not dry overnight like the others did, never did dry over a few days. I never did it again intentionally to see if that was indeed the result, but never had it happen again.
I always see posts stating that even a little residue of oil, grease, butter, or shortening on utencils or bowls will ruin royal icing. I have not, however, come across anything stating what the oily substance actually does to it. If anyone knows, please let me know!