sounds like you are not killing the fungus, mold or whatever is back there before you re caulk and its only coming back. I would do this, get on a tight regiment of spraying pure bleach out of a old windex bottle and saturating the area. do this for a couple of days making sure that all growth and any dark areas are clean and white. Then stop this after three days and let it dry out, no showers or baths and let it be void of all moisture. Then do the recaulking using GE brand silicon 100% caulk.