Sounds to me like the caulk needs to be replaced. Just noticing all the cleaning solutions you listed as having tried, and thought you might be interested in what I have done. I actually converted my home to better, safer cleaning products, which do not contain any bleach, ammonia, phosphates, or other harsh chemicals. My home is cleaner and my family is healthier. I love not having to try not to breathe while cleaning my bathroom because using these cleaners in a small space does not make me feel dizzy or ill. My new cleaning products are actually less expensive per use than the grocery store brands so I'm thrilled with my discovery. I do not sell the products, but if you are interested in "going green", converting your home and shopping direct, then I'll be happy to tell you how you can do it too!