There's so many ways we can "not know" how to apply our knowledge to something. Given a situation where X is useful, you could:

1. Not realize X is a thing at all
2. Know about X, not realize it applies *here*
3. Realize X is useful, but not know how to apply it https://twitter.com/sliminality/status/1378809330707787776

Often with topics "forced" onto practitioners (math and theoretical CS come to mind), they're taught the topic but not about (2) or (3). Knowing algorithmic complexity ≠ knowing a certain problem is tractable with it ≠ knowing how to apply that knowledge to the problem

Sometimes people are surprised at how often I "use math" as a programmer. It's not because I'm doing anything particularly cool, I just have a lot more practice with "seeing the math" in day-to-day problems