Ungrateful? I don't think so...

I learned that not everybody reacts to smile or compliment in a visible way. Sometimes it takes some time before you see how it helped. Sometimes they don't respond at all.

But that doesn't mean that they aren't glad to hear that. We can't read people's minds. We don't know, what's going on there.

Maybe they just don't know, what to say. Or they're shy. Or they're shocked because they didn't expect that.