jessabella - after nappies have been used for a while, detergent tends to build up in them cos machines rarely wash 100% of the detergent out. That can make them less absorbent and smelly, so after a while you need to 'strip wash' them to get the build up out. Either wash your nappies with a dishwasher tablet in (yes, the kind you'd put in a dishwasher for your plates and cutlery!) and rinse a couple of times, or use the full dose of detergent recommended on the box/bottle and then keep rinsing the nappies afterwards until no more detergent suds come out of them. Easy!
Lots of people do this regularly to prevent the build up getting to the point where it is noticeable, say once a month or every 6 weeks.