|That is hard to answer without looking at the project.
Most of the time the reason it cracks and falls out is there is some kind of movement there.
What did you tile over?
What is the space between the tiles? Did you use the right type of grout? There is sanded for over 1/8" gap and unsanded for less than 1/8" gap.
To repair what is falling out, scrape out what is loose or damaged with a grout removal tool. Then regrout and clean with a sponge. Make sure all is clean and seal all the grout with grout sealer.