Part of my confusion was that it appeared to work in situations that had a pi in the answer. So if the answer was "(-pi/3+2)/7" and I tested an answer of "(5 pi/3+2)/7", it gave the answer hint. If the answer was "2/7" (strictly "(0 pi+2)/7") and I tested the answer "(2pi+2)/7" there was no hint.
I presume the constant pi was making the resulting value act as Real and take a periodic property even though it could still be entered in a strict fraction context.
I'm happy to see that the major thing I was missing in my attempts to make a subroutine was the &.
I just learned about both the periodic property and AnswerHints, so I think I'm about to revise a lot of questions. It will be good to be able to specify whether the hint is shown in Preview (even though that had not been my original intent in doing this).