It's someone who may appear quiet and reserved, or even arrogant but actually just doesn't know what to say or what to talk about. They're not necessarily shy and not dumb.
I tried words such as 'reticent' and 'taciturn' , but those are more like "not wanting to talk" than "can't think of what to talk about". 'Inarticulate' is not it either, it sounds like something is physically wrong with them. 'Ineloquent' is more of not being clever with words, not able to persuade others with words, which is not what I'm looking for.
I don't know if there's even a word for this. Anyway, I just want a way to describe this trait using as few words as possible.