A contractor tiled my bath a few years ago, and less than a year later the caulk between the tile and tub was cracking. I taped over it with some plastic sheeting and finally getting around to doing it.
The caulk is hard as a rock. I got some Caulk Be Gone that's supposed to turn it into soft putty. 5 hours later it's still hard as a rock. I even got a special tool to take it out and smooth the new caulk in. Doesn't do anything.