koi disappeared again..

hi all,

i live in the US pacific northwest, and I began feeding my koi the
appropriate wheatgerm feed a couple months ago when a local store
suggested it'd be warm enough to start feeding them.. they came up
every day and ate and all was good.

but 4-6 weeks ago they stopped coming to the surface, and i haven't
seen them since. it's as though they're hybernating again.

should i be worried?

thanks