I thought I'd give my 2 cents here with regards to smelly nappies. I had a customer that after bringing them out of storage and wanting to use them on a second baby they started getting that awful smell from build up. They tried everything and I was in contact with them on several occasions until they got this tip and they are delighted now that they are using them with no smells whatsoever on their second baby. So I know this works and it was a severe case of smelly nappies to the point that they said it stang their eyes when they changed the nappies; so it was an unbearable case really:
1. Soak 12-15 nappies in hot water between 50-60 degrees Celsius or 130-150 degrees Fahrenheit, for several hours, with no detergent or anything added. Every hour or so, you can manually agitate the nappies somewhat to facilitate the process.
2. After soaking for several hours, do several hot wash/cold rinse routines in your washing machine, with no detergent added. This will ensure all the build up is removed. You should do at least 2-4 of these cycles, if you think you need more, go for it, rinsing never hurts!
3. Do a regular wash routine: initial cold wash, hot wash in water between 50-60 degrees Celsius or 130-150 degrees Fahrenheit with 1 Tbsp of detergent, and 1-2 final cold rinses.
4. Dry like normal.
This will get your nappies back to baseline and should get your stink under control. You normally will only need to do the regular wash routine as written out in step 3 to keep them maintained. Make sure you are only washing between 12-15 nappies and your hot water is between 50-60 degrees Celsius or 130-150 degrees Fahrenheit, both of those factors will make a huge difference, along with only using 1 Tbsp of soap.
I hope this helps anyone out there with smelly nappies!