My guess is you wash mainly at low temperatures (less than 60) and use liquid or bleach free detergents.
This allows the mould to build up. You need to buy the cheapest washing powder (MUST be powder) with bleach and run your machine empty on the hottest wash.
This should improve the mould, but may not remove it all together. You can add a handful of soda crystals to the drum if you like, before you start the hot wash cycle. You need to do this maintenance wash every month or two if you don't want to change your washing habits.
Leave the machine door slightly open too between washes - this allows the seal to dry out thoroughly.
You may have to replace the seal if it is too bad. :(