Maybe your problem is thatch:
Thick thatch makes mowing very difficult.
As thatch builds up, mowing height actually increases
above the soil line, and the turf becomes very spongy,
allowing the mower to sink into the turf and scalp the
lawn. This results in an uneven appearance and often
a mottled brown and green surface
http://manatee.ifas.ufl.edu/lawn_and...h-in-lawns.pdf