I have a maple tree(40 yrs. old) in the front yard. When I bought the house 8 yrs. ago, there was grass growing under the canopy of the tree. 3 yrs. ago the grass died. The tree was trimmed to lift the canopy, then I tried reseeding, didn't take. Last year I put down top soil and planted grass specifially for shade. The grass grew until the 1st mowing then it died. I have a small area of Blue Rug planted below the tree, 10 feet away. What might be causing the grass not to grow?