Possible causes & solutions:
Excessive use of detergent (enzyme based) during low temperature cycles, use less detergent & avoid cold cycles, carry out maintence washes regularly.
Excessive use of fabric conditioner, use less concentrated fabric conditioner.
The appliance door is kept closed after each use, always leave the door ajar/slightly open after every wash to allow fresh air into drum to dry & reduce mould/mildew build up.
Refer to instruction book for further information.