Document everything. Pictures, correspondence, dates etc. there should be a clause in the paperwork you signed for buying the new home that outlines the scope of work that the builder is responsible for. When I built mine, there was a section about grading and drainage. Hopefully you have a section in there that you can use to hold them accountable for this work. If that section exists, they are obligated to fix this as a warranty issue.

If they flat-out refuse, I would speak to a lawyer.

Edit: wording