I had 12x12 ceramic tiles installed in my kitchen back in February. I've had the contractors back twice to replace tiles that I found to be high on one of the edges. I'm not sure if the guy used spacers - I am finding high sides on several tiles. Anyhow, the newest problem I am having is hairline cracks along the grout lines. Some of the cracks go for a long way and some are just the length of a tile. I believe the floor was installed over hardibacker that was placed over the original linoleum floor (house was built in 1975). Are these cracks typical of ceramic floors? Do I need to have the contractor back to either replace tiles or replace grout?