This will probably count as the least useful answer ever, but I have this problem from time to time. It starts apparently without cause and then stops equally without apparent cause. I don't think it can be anything to do with my customising because I can be having problems on my PC at home but not at work and vice versa (btw I am the boss so it's allowed!).
I have a smart phone but I never ever ever ever use it for MN purposes so I can't comment on that!
Ultimately I find waiting it out puts it right, and not much else.
I'm on an iPhone (using desktop site though not app) When I had previously clicked on a thread, the thread title would com up a different colour so that I knew I had read that one It's not changing colour anymore though and that's in active and topics