Don't worry. There is nothing wrong with your brewing technique. This problem is in fact a characteristic of the ingredients you are using and is readily solved. All you need to do is to use quality branded beer kits and malt based additives.
What you are trying to achieve is a beer which, after time in storage, will present a tight, creamy head that will cling to the inside of the glass to the very last drop.
Helpful hint :- keep your beer glasses clean and free of detergents and greasy foods.
The easy way to keep them clean is by briefly boiling water in the glass, in a microwave oven, then storing the clean dry glasses in the fridge (not the freezer).