Not blindingly obvious at all! The little progress bar sometimes does odd things, especially when it’s travelling in a downwards direction.
When it says your Nightmares have fallen from 6 to 5, what it actually means is that they’ve fallen from 6-point-something to 5-point-something. Say for the sake of argument, from 6.2 to 5.8.
The next time, they drop from say 5.8 to 5.4 - the same amount. But because that hasn’t pushed you over a whole-number boundary, it just says "Nightmares are dropping", and the progress bar animates in a different way.
So, in fact, they’re going down in perfectly consistently, it’s just the way it’s displayed that’s a bit funny.
NB. I’m deliberately simplifying - the "point somethings" don’t work in quite the way I’ve implied here. If you want to know the details, the section on "Change Points" at http://fallenlondon.wikia.com/wiki/Beginner’s_Guide explains exactly what’s going on.
Cheers
Richard