I found a strip wash did the trick too. I have only had to do it the once in the 15odd months of use! I think I just did a hotter, longer wash with extra rinses. Me thinks it might be time for another.

Regarding detergent quantity: I found my nappies were smellier when using less detergent. I now use just under the normal scoop but changed brands and that did the trick.

I always do a prewash with a few drops of tea tree oil, a long 60C wash and regular rinse with vinegar on a cotton cycle.