I've found that if I let my grass grow to 3-4 inches, some of the grass doesn't always stick straight up and goes diagonal when the mower goes over it. Then the grass it's covering doesn't grow as well, creating bare spots.

This is probably because the previous owners of the lawn didn't take great care of it and there's already some thin spots.

So I have to cut it to about 2 inches to allow all the grass a chance to grow.