Jen, I am not too sure about the nappies you're using but as far as I know the regular washing detergents are not meant to be used on MCN because of the softeners and enzymes used int he regular powders. The EcoStore or Planet Ark powder are the better ones to use asn they have NOTHING in them that will cause build up or even start breaking down of the PLU fabrics.
It MAY mean you have to buy a different powder for your nappies but you will use a lot less powder in fact we have only bought 6 lots of the Ecostore one and our DD is 21 months old and we wash every second day. You are also meant to use only 1/3 to 1/4 of the recommended measure for the washing powder.

I actually wash my nappies twice every single wash. Once with a teeny bit of powder and again with nothing to ensure there are no suds.