Randy offered some good guidance.
I've done several of these jobs, with significantly varying results. Generally speaking, beyond the condition of the sealer, if the tiles look to be in poor shape with lots of discoloration, RUN. Trust me...I've learned the hard way.
If the tiles are in decent condition, but the sealer is simply failing, the job is salvageable. The sealer could be failing due to normal wear and tear or a poor application. What I've found is that if the previous sealer is not completely stripped off, a new sealer application will not bond properly and will fail, similar to your picture. In situations where I've had positive results, I've used products from Aldon. Their website has a lot of good training info on these tiles and how to use their products successfully:
http://www.aldonchem.com/mt-mexican-tile.htmGood luck.