There could be 3 reasons:
A build up of food deposits/residue may have formed on the pan which may originally be the result of an item of food being allowed to overcook in the pan.This can normally be cleaned by filling the pan with water and washing up liquid and leaving overnight. Then use a paste made up of bicarbonate of soda and water rubbed gently over the pan with a non-abrasive sponge (a non-stick scouring sponge).The non-stick coating should then be reconditioned with a little cooking oil wiped over the surface with a paper kitchen towel.The frying pan may have been overheated causing damage and discolouration to the non-stick coating. When pans are regularly overheated this will lead to a weakening of the non-stick coating.If cleaned in a dishwasher, after each use re-coat the non-stick interior with cooking oil to prevent the non-stick from drying out and ensure the best non-stick performance.