YOu need to determine if the problem is the result of a poor grout job or movement in the tiles. My first guess is that if the grout is bad, you'd have the problem EVERYWHERE. If you don't, then it's most likely due to movement.
There are 'levelers' today that level out uneven floors. I know you say 'concrete underneath' so I'm assuming you AREN'T referring to cement backer board. If the tiles are installed directly on concrete, there may be an adhesion problem between it and the tiles and the glue. Have 2 or 3 pros come out to assess the damage and give estimates. By the time you talk to the 3rd contractor, you'll have learned a lot!