Maybe do a cold wash? I was always told that hot water sets a stain and if that's true, then maybe it sets a smell too?

BBH mentions the use of Dettol...'you can use it in your rinse cycle by half filling the fabric softener dispenser with it and letting your machine wash as normal. Use it only when you feel your nappies need a freshen up'.
or
'you can do a hot wash with NO detergent, but 1/2 cup of bicarb soda, then 1/4 cup of vinegar to your rinse cycle, then keep rinsing until the water is no longer sudsy'.

Hm, good luck!