Perhaps the oil or fat isn't fresh. Make sure you change the oil or fat regularly. Sieve the oil or liquid fat from time to time to keep it fresh longer. Make sure that the type of oil or fat you are using is suitable for deep frying. Use frying oil or liquid frying fat of good quality. Never mix two different types of oil or fat. Also check if the permanent anti-grease filter is perhaps saturated.
To clean the filter, remove the lid (except HD 4282) and clean the lid and filter in hot soapy water or in the dishwasher.
For HD 4282/ HD4283 please check if the carbon filter is saturated. If it is, replace it.