This spring I planted grass underneath and around a tree in my front
yard with a sun/shade mix. The grass sprang up quickly and seemed
healthy most of the summer. Over the last two weeks the grass
has almost completely disappeared in the shaded area leaving me with
just dirt.

Is there a particular type of seed I should use? Is it too late to
re-seed the area this year (I'm in Maine)?

Thanks for any advise.