Where are you trying to caulk?
This article is helpful: https://www.hunker.com/12521461/how-to-caulk-over-grout
Key is cleanliness. If there is soap scum, mold, mildew, water deposits on/in the grout, then that needs to be removed & completely dry first.
You must scrub the grout really good and remove any loose pieces before caulking
Why not just regrout?
You don't. Caulk will not hold to grout for long and it won't look good.