Does you home have the steel cables going across the slab and bolted to the other side. Called Cable Lock System?
Was the ground dug out and properly done, water, sand and back fill?
Are the cracks on one particular side or just general?
If one only one side, that means that that side is failing due to improper foundation construction.
When a crack developes in either new or old foundations, you do have troubles, either now or later.
These foundations are built with a system of rebar and deep concrete and hopefully a cable lock system.
When it cracks, there is a problem, whether small or large. this is now your weakest link and all movement will now occur on this "fault line" if you will.
Listen to the excuses the builder will give you and see if they will do something now for you. Take pictures and document all conversations and excuses given.
You do have a big decision to make. Do not let them persuade you or convince you that this is normal or expected, when they are gone and your doors and windows and flooring heaves and dropes, they will not be around. So, this is something going on that is suspect.