I have a Mira plumbed in shower which is 18 years old. The shower works fine except for a poor seal where the hose connects to the shower outlet. It leaks water constantly and I cannot get it to seal properly. The rubber washer is in place, I have tried ptfe tape on the threads but that doesn't help. I have tried new hoses, metal and plastic but the leak persists. Am I doing something wrong? It seems such a simple thing but I can't fix it. Any ideas anyone?