The cutting blade is angled like a propellor and like a propellor if gets fouled or coated in grass 'mush' it will not deliver a blast of downward air to make it hover properly.
Turn it over clean the blade on both sides until you have shiny metal. Now clean the blade housing until you can see all that orange plastic.
Turn it over so it is the right way round. Now put it in your car and bring it round to my house and give it a one hour test on my lawn!
GB in Wales
February 2013