How do the same atoms sometimes create emission lines and other times create absorption lines?

Answer 1

Hi. When an electron absorbs just enough energy it jumps to a higher energy state (which we call an orbit, but it's really just an energy state.) This 'absorbs' energy creating a dark line in the spectrum. When the electron returns to a lower energy state it emits a photon at this same frequency.

Answer 2

It has to do with their energy states.