Why does "...at once" sound fine but "...right now" doesn't?

A non English native colleague asked a questioned today that I couldn’t answer clearly. The only thing I could come up with was that it sounded strange. The sentences:

"They liked him almost at once."

and

"They liked him almost right now."

I mentioned that if almost was taken out, either would be fine (at least it sounds fine to me).

Can anyone give a detailed explanation as to why right now sounds weird?

edit: Also can anyone give a grammatical explanation why?