The machine readily accepted another one from the same year on the first try, but this one in particular it wouldn’t take. Anything special about it that I’m missing?
Luck of the draw. Perhaps one self-checkout is improperly calibrated as opposed to the other. One should expect this with paper currency because as the currency is used and ages, its visual properties deteriorate and the currency scanners begin to reject older bills. Coins not so much as they wear much slower.
The coin acceptors (vending machines) are based on several coin attributes that really don’t change much as a coin wears. The older coin acceptors used to have a bounce bar to help differentiate between coin values and coin legitimacy.