It sounds like the center roller of the deck is flattening the grass and there isn't enough vacuum to pull it back up to cut it all. Check your deck to be sure the front edge is 1/4" lower than the rear. Did you add air to the tires or adjust the deck before this started to happen? Did you adjust the height of the rollers?
Haven't done anything... I need to check the deck height to see what the measurements are.
That center roller idea sounds logical... I'll check that out also.