My guess is that it's a budget issue - I don't know the terms of their contracts, but that's my guess. (Why they'd bring in Haylen when they were cutting screentime for series regs due to budget constraints is a different can of worms, but she seems to be gone, so there's little point debating that now.)
Lindsay has missed several episodes, but she's also been relatively prominent in some of the episodes she's been in - it seems like a fair trade to me. If Anna, Bob and AJ are going to be missing some of the time, I think it's only fair that they get a bit more focus when they're around. (Sid was prominent in the lingerie football episode, so hopefully Lindsay and Adam will get a similar chance to shine at least once this season.) As long as that continues, I'm not too concerned. It sucks, but eh. Hawkes is there every week and doesn't seem to get much focus, so I'm all about balance at this point.