I had the same problem with a Bathfitters system.

1. Cut/clean off the old caulk. Use a razor knife, or the like.

2. Clean with alcohol to get rid of any grease and prep the surface.

3. Re-caulk with the self-vulcanizing white silicon caulk. The stuff
from GE seems to be best in my experience. It is the stuff that smell
like acetic acid.

What appeared to happen to me was that things settled a bit, and
pulled away. I have the same problem with a new shower in a house
about 2 years old. Dug out caulk/grout, and recaulked, as I believe
all the settling that is going to occur has now pretty well happened.

Your milage will vary...