Sun, bicarb, and stripwash are the best things for smelly nappies. Boys also have stronger Pheromones than girls so they are likely to get smelly a bit faster with boys. Try popping some bicarb in the bucket if you are leaving them for a few days and hang outside whenever possible. I find they smell a bit faster when they only go in the dryer. They can handle up to 60 degrees wash which is higher than what most HWS go to nowadays. and low in the dryer. How many prewashes did you do Kate? I find that if I have only done 1 or not prewashed they get a bit smelly because if the manufacturing process, because the microfleece hasn't fully washed in, and the bamboo needs to get a little more absorbency.

I reckon try the sun and bicarb and if no success then strip wash.