If reroll works anything like PL (which is not a crazy assumption), the reason you don't see a reroll every time you get a drop with 100% reroll is that you only see successful rerolls. A reroll just means you get a second roll. If the second roll results in a better drop, then it replaces the first roll and you see that you rerolled. If the second roll is inferior, you don't even see that it happened.