probably low water
I haven't been to the Flatbrook since that warm spell in February, but I have to think it is running quite low with the lack of rain. Which means the stocked fish are going to be concentrated in the deeper holes. And I think most of the river is still open to bait/spin so your going to be competing with that unless you stick to the Blewett.
The Flatbrook does not do well in low water, but if we get a decent amount of rain over the next 2 weeks the first couple weeks of May are prime time for that piece of water.
Anyway, as far as flies.......Hare's ear, PT, and sparkle pupa all with bead heads are good producers. Go small with low water, 12-14 even 16. And while you're there pick up some of the bait containers left behind and remember Roy King. :beer: