If you are not getting a rectangle added to the BoundingBoxes which has an X of 608, then you need to look at your Blocks array, because that is the only thing stopping a rectangle being added when i*32 == 608, or i == 608 / 32, or i == 19 given that you have checked ChunkWidth and are sure it is 20.
Put an "else" clause in there:
if(Blocks[i,j].Material != BlockMaterial.Material_Air)
BoundingBoxes.Add(Tangle);
else
{
}
and put a breakpoint on the '{' - that will confirm it when the debugger hits it and allow you to check the array content.
You probably should be using Blocks.GetLength(0) and Blocks.GetLength(1) instead of ChunkWidth and ChunkHeight, or at least checking that they are within the limits of the Blocks array.