I am putting tile in my kitchen and have grouted the tile but I find that in some areas the grout continues to crack even after regrouting that area. What am I doing wrong?
Cracks mean movement. Could be from numerous sources: subfloor too thin, improper thinset, no decoupling layer (cbu or membrane, or really expensive flexible thinset), joists too springy, you used mastic rather than thinset (bucket pre-mixed verses powder you mixed up), not enough coverage, you walked on the tile too soon after setting, etc.
A good place for tile info is www.johnbridge.com. They eat, drink, and sleep tile there.
This should be in the remodel forum not plumbing, but I would guess bad backing, the tile is only as good as what is behind it and the glue you used. Corners will always crack and must be caulked.