Ebony Knight
Lore Sword
Ok so while traveling around in blackreach Lydia has been a real pain for me and I've come upon a few observations. First of all if you notice that Lydia is missing, immediately start backtracking. If you put too much space between the two of you she will just stand still until you return and in some cases WILL NOT CHANGE ZONES WITH YOU. Keep an eye on the girl or she is liable to go missing. Another thing i noticed is she will also stop moving altogether regardless of distance. You can tell her to follow, wait here, pick something up, Fuus Ro Dah her, she will not move. The trick I found to getting her to follow again is run into her at full sprint a full times. Eventually she will get staggered and start following again. Don't know if this is the case for other companions but it's worth a shot. Hope it helps.