Sandy's are awesome My nappies get a bit smelly sometimes, regardless of how recent a strip wash was. I can't really work out why. I can't pin it down to a particular detergent, as I change pretty often. I'm really cautious about not using too much detergent, and I think sometimes I actually don't use enough. If you use none, I think a 40 degree warm wash is the best.

Time in the sun definitely helps. A lot. If no sun, then even just fresh air is really good.

Strip washing: one drop of dishwashing liquid or hand soap in a hot (60 degree) wash. Then rinse, and rinse, and rinse, until all the suds are gone.

Good luck!