A reader wrote to Ask a Manager in 2018 about spending two hours pulling together information for a coworker's meeting, sending it over, and never hearing back, not a thank you, not even a confirmation it arrived.
Why the favor did not register
The advice columnist's read was direct, it is rude not to acknowledge someone going out of their way for you, but it is also possible the coworker genuinely did not register that this was outside the letter writer's normal job, because nothing about how it was delivered signaled that. A favor performed quietly, without the small verbal marker that usually accompanies one, can be completely real on your side of the ledger and invisible on theirs.
Who this catches more often
This lands harder on someone who tends to help without narrating it, doing the extra work and moving on rather than mentioning it later. The generosity is not the problem. The absence of the one sentence that would have made it visible is.
Where to start
This is a case for one plain follow-up, not for turning every favor into a performance.
If you did something that genuinely helped someone, a short mention later, not at the time, files it properly. "Glad that worked out" said once, weeks after, does more for how the relationship is remembered than the original favor managed on its own.
Reading which favors are actually worth that follow-up, versus which were small enough not to need one, is a judgment only you can make about the specific effort involved. What this gives you is the one sentence worth adding, after the fact, so it actually registers.
There is a specific way to mention a past favor weeks later without it sounding like you are calling it in, and it is worth having ready for the next time you helped someone quietly.