Flooring Tile - Re-grouting

Sections of the grouting in the ceramic tile in our mudroom have come loose. Although I have the original grout to use, is there any prep work (other than cleaning) that should be done before re-grouting? For instance, is there a primer that will increase the adherence of the new grout to the existing tiles and original grout that is still in place?

Thanks!

You are one step ahead of yourself. Before you decide that regrouting is ok, you need to find out why the grout is cracking in the first place. Grout usually cracks because there is too much movement in the substrate below. Either the joist system or the subfloor may not be strong enough to support a tile installation. Regrouting will likely result in the grout cracking and coming loose again in the near future. Find out why your grout is cracking before you regrout.

Things you need to look at are, what are the size of the floor joists, what is there on center spacing and what is there unsupported span from below? What is the subfloor, plywood, osb and how thick, and how many layers? Was a cement backer board or isolation membrane used? If cement board, was the board bedded in thinset before it was nailed or screwed down? Was thinset, or mastic used to adhere the tile? Based on your answers, maybe some other questions as well.