Hi Sharon,
I'd check your supplement bottle to see if it says it is the sulfate variety of G and C.
I should mention, the original studies showing benefit were done with glucosamine, not including chondroitin. But they usually come together in supplements and that's fine.
Regarding your knees, if their trouble includes osteoarthritis, G/C is worth a try--my Mayo orthopedists say they hear this from osteoarthritis patients all the time and advise staying on it. Start by getting a good diagnosis if you haven't yet.
Joint problems do their best when their area is kept as strong as possible and obvious aggravating factors are avoided. The problem is, to avoid aggravation in a troubled area we tend to not get needed exercise for it, and the area becomes deconditioned and even more problematic. A trainer can be helpful in selecting appropriate exercises. If weight bearing exercise, for instance, aggravates knees, often a cycle will work.
For sciatica, a good diagnosis of its cause, appropriate treatment and exercise training are all important.
Wish I could recommend more supplements, but the studies aren't supportive.
Best of luck to you Sharon!
Scott