Ruby had the same problem, only it was both of her eyes. Out of the 4 little openings of the tear ducts, 1 was okay, 2 were underdeveloped, and 1 was nonexistent. They put her to sleep and used little catheters to enlarge 2 of them and made a surgical opening for the one that didn't exist. They said they flushed out so much mucous and junk in her ducts, and she's looking much better. She was pawing at her eye so much, so we were just really afraid that she would injure her eye. She did great with it, and was probably on the same drops as yours before and after the procedure. I thought she'd be groggy like she was after her spay, but she was completely normal and didn't miss a beat. She still gets the runny eyes like any Cavalier, but they're much better than they were. Good luck with yours!!!
Jessica, mom to Ruby, 1 yr. old blenheim