Do you prewash the nappies at all? Do an extra rinse at the end of the wash? If you have never stripped your nappies they are well due one. You will hopefully notice a difference in absorbency afterwards.
The best way to wash nappies is a cold prewash with no detergent followed by either a 40 or 60 degree wash with (depending on how soft/hard your water is) anywhere between 1 tablespoon up to half recommended dose of biological washing powder followed by an extra rinse cycle to ensure that all detergent is rinsed from nappies & inserts. Washing powder is better than liquids & gels as they cause more build up.
To strip wash, do a cold wash with a full dose of washing detergent, then do a 60 degree wash with no detergent. Then do several extra rinse cycles.
For more info read this thread viewtopic.php?f=19&t=23823
Then do a maintenance wash on the machine as outlined in the thread linked to clear the machine of any build up.