You are simply chasing symptoms instead of curing the problem. Your cat has a sinus infection. the mucous in his throat is from the draining sinus. The solution is to get rid of the sinus infection. Dexamethasone suppresses his immune system, whereas you should be strengthening his immune system. Since the Hydrastis worked initially, get a higher potency Hydrastis, such as a 30c, and give that twice a day. You might ask your vet for Clindamycin, an antibiotic carried into the sinus by leukocytes. You could also try giving a good immune stimulant.