In Spring I've open fed pollen sub for several years now just as soon as I notice the willows budding out. Some years the bees are all over it and some years they ignore it. I don't have need to check for stores as I use MB's dry sugar method which pretty much assures plenty of sugar stores if honey stores run low. I use pollen sub (and dry sugar) as insurance only, and is why I only OPEN feed it.
I 'suspect' that during the times the pollen sub is ignored also coincides with the times SKUNK CABBAGE is plentiful as it will bloom well before our willows, even in the snow, actually melting an area around it.
My bees always find it and LOVE it, stink and all 8-)