we had this very same problem with our bumgenius nappies, not sure what type nappy you have. feel your pain, me and the dh were about to give up too, after all you don't want your bub and your house to stink of wee;-)!
came to the conclusion after much reading around it was prob detergent build up which apparently lets yucky yeasts grow in the nappy making them stink when peed in. so the consensus was stripping was needed
what we did to strip was:
1.soaked all the nappy inserts in a dilute bleach soln (whatever domestos says the concentration for soaking is),
2.washed inserts and outers at 60, then kept putting through rinse cycles til there were no suds visible (takes several). 3.then turned the outers inside out and hung the whole lot outside (pref sunny and windy).
now whenever we wash the nappies we:
1.separate the insert from the outer before washing
2.put through a cool cycle first with no detergent
3. wash at 60' with small amount detergent non bio
4. double rinse cycle or more if lots of suds
i tend to stick dirty nappies straight in the machine until the next wash and close the door. saves any niffs permeating the house. although urine niffs now gone poo still pretty smelly
if i'm doing a wash every third day, i would do a rinse cycle on the second day just so the nappies aren't sitting for too long soaking in urine. bumgenius recommend occasional bleaching to keep nappies fresh. bio detergent and vinegar suggested prev might work but depending on the nappy they might strip the waterproof inner (eg bumgenius says def don't use for theirs) so might be worth checking out the website of the brand you're using.
we did all this over a month ago and no problems since, will just repeat the stripping if problem happens again. sorry to be so longwinded but, without being dramatic
, i was nearly in tears over this so thought it might help you