if it was just one cat with recurring probs then i'd be wondering about blocked tear ducts.
more than one..and never clearing properly i'd ask the vet about chlamydia, now my brain cells are sketchy on this as we didnt see it alot and my old practice wasnt really a "breeders" vets which would be more aquaint with it but im sure that has symptoms in the eyes, might be worth before anymore treatments your vet takes swabs.