A person can be grateful and still need to explain what would work better next time.

Gratitude often gets treated as a gag order. Once someone has helped you, saying that part of it did not work feels like an insult to the kindness, so people accept the wrong-sized help repeatedly and quietly stop asking. The helper meanwhile keeps doing the thing nobody wanted, believing it was welcome. Both are being polite, and the arrangement gets worse for it.

Thank someone properly and then say the one thing that would make it more useful next time. Put the two in the same breath rather than saving the second for later.