I've stashed about 50 rerolls over the last few months, and I went to use some on my horses, and it just didn't work? There was no coat or pattern changes, it looked exactly the same as before. I tried it on different horses, and I can it to switch a coat from mahogany Bay to fun and back, but no new coats besides that, no new markings. Am I just doing it wrong, or is it actually broken?

I rerolled this Mare just today. She started as Dunskin, post reroll, no change. No different marks, coat, nothing.

A dunskin is going to stay a dunskin. It is also possible to reroll the same coat and markings. But a horse isn't going to get tobiano if it doesn't have the gene.