Richard, didn't you answer your own question? If the blades get "set' the pump doesn't work. It's also good practice to re-learn impeller replacement techniques on a regular basis and to assure that you have spare impellers. We have a spare shaft with a new impeller, ready to go, all the time.

Richard,
I've read in several places that the non-toxic antifreeze most owners use is bad for rubber impellersas it tends to harden them. What I normally do is to take the impeller out of the pump after the engine is winterized, and put it on the nav table along with the keys. In the spring I reinstall the impeller and it works perfectly. Just a thought.

I have never taken out the impeller over winter on my 35BC engine and have never had a problem in Spring.

Ditto,
I do not remove my impeller over the winter. Unless you sail in an area with a lot of silt in the water, or forget to open the seacock, the impeller will last a long time, thanks to modern polymer technology.

Turning the engine over by hand or with the starter motor before you start it in the spring is never a bad idea, as it pumps the oil around, and give the blades a chance to lose their set, so that they will be able to pump water when called upon to do so.