Some models have a part called a grease shield, which starts to droop after a few years of operation. If the shield droops or sags, it can interfere with the even dispersal of microwaves as the unit cooks. You can order this microwave replacement part for under 20 dollars. Follow the instructions in the manual for your particular unit and do a safe install with the power plug disconnected from the wall socket. A more detailed list of steps isn't possible here as many units have differing features and requirements for installation. One feature common to all microwave replacement part installations; to buy replacement microwave parts, you will need to have your model number handy before ordering. Some websites require you to search by model number or name brand at the very least to find your microwave parts.