It appears that glucosamine+chondroitin is most effective on knees when it hurts a lot, not just a little.

Efficacy of Glucosamine and Chondroitin Sulfate May Depend on Level of Osteoarthritis Pain | National Institutes of Health (NIH)

I was told to take it for a mildly arthritic thumb and haven't really felt any improvement. I guess I didn't believe in it enough.