I have similar problems with my whippet Rickie whose hind end is in decline. Like Alan he is on Cosequin DS (well actually now Flexadin, pretty much the same stuff). My holistic vet suggested a product called Vetri-Disc by Vetri-Science. I am giving him half the recommended dose because some of the ingredients overlap with the Cosequin/Flexadin. Too soon to know if it is making a difference.
We are not doing acupuncture at the vet's suggestion, but are doing gentle chiropractic every few weeks.
I am also getting Rickie to go up to the second floor for every treat, to try and increase the muscle mass in his back legs, and it seems to be helping (we started this week and are doing about 10 special trips per day, a few at a time, spread through the day).
Hugs to Alan.