I think I found the problem.
A google image search netted me an exploded view of my multi-port valve, and I noticed there is supposed to be a spring on the shaft; a spring which was missing when I disassembled the valve to replace the o-ring. I imagine it died of old age, and I'll maybe find it (pieces, anyway) when I change the sand this year. So, eureka, maybe? Fingers crossed.
Thanks again for your help, John. You may have not directly pinpointed my issue, but you led me to actually do some research, to stop sounding like such a newb trying to describe my problem to you.