I stowed my Sawyer gravity filter over winter w/o doing the bleach routine. After the last fall trip I backflushed it and left it on a sunny windowsill a couple weeks to dry.

This spring I took it on a trip and at first could not get any flow whatever. Gradually it improved and was flowing maybe half-normal at the trip's end. Back at home I backflushed it until normal flow was restored.

So I'm guessing I had bacterial growth over winter that clogged it, and I should have done the bleach thing before drying and storing it. I'll do that in the future. I'll also do a backflush and test flow before using it at the season's start.

Cheers,