Seems to me there are often times when a certain member or two aren't on the show just for reasons that they didn't need to be there for a certain day of filming or couldn't be there for some reason. I'm pretty sure they end up filming multiple episodes a lot of the time when they come into the studio, so her missing on multiple episodes could've just been from missing one day of filming. Maybe she had a photo shoot or something else to film that day? I think Suu-chan used to miss a number of episodes and events because of that. Ruri is pretty busy in the photo shoot department, she seems to end up in at least a few magazines a month.
I've also noticed Leia being really sick lately on TV performances and Idoling!!! episodes, yet she still shows up to work! I think they just leave her mic off cause you can see her sniffling or doing other things that generally create sound, but you don't hear it.