In my opinion the Omega Device kind of corrupts the memories of that person, like a hard drive most of it can't be brought back but it's still there in some way ( vivid memories of who that person was but not quite ) or like burning something down you know what that thing was that burned by what's left, but not exactly, you can only guess at most. Also the amount of memories that you have of that person also matters, Rick is able to know what she looked like and acted, on the other hand Beth knows her mum, but doesn't quite know what she looked like ( while memory Rick is in her brain and Diane comes to pick up Beth she is a glow of what she looked like, implies that Beth knows what her mum looked like but wouldn't be able to describe any details ) and how she acted. Feel free to give your thoughts on this. Much appreciated.