Question

Why did ammunition arriving at Amelia court house allow the union to catch up to Lee

Answer 1

Lee stopped at Amelia Court House to wait for the provisions that supposedly be in the area. Unfortunately, it did not arrive. They waited and made other plans of gathering provisions at a near area but they received nothing. 
They left the Court House and discovered that the Unions already reached them. Lee made some strategies to go around other areas, but despite all of the plans he made with his troops, they did not succeed.