Reply #6: By yer description somethings going on in the bathroom.

[Wash. state Desk Jet]

What kind of caulk did you use.? And are you using 100 per cent silicone caulk? The stuff runs about $5.00 a tub.
Do you have any leaks around the valves or behind the walls perhaps? What about the bath ceiling fan does the bathroom have one, if so try running the fan all day . After four hours if that stuff is not setting up you have a sub standard product or water /moisture is causing the problem. Did you try putting a small space heater in the room? Try bringing the room temp. to at least 72 degrees run the fan and get some decent caulk.And you might invest in a moisture/mold test kit at a cost of about $10.00.

I wonder if sheet rock compound would cure as it should in that bathroom. You got a heat gun or a hair dryer? You up to a little experiment? By the way is there a window in that bathroom? Is there sweating on the tiles or walls or perhaps a window?