I have the same issues, and I dont know what to do either. Its mainly his NN's. DS did one wee in it tonight and DH and I both thought that he had pooed it was that bad.
I do a cold pre-rince, then another pre-rince on warm followed by a huge wash cycle with heaps of rinces at the end. I air the nappies and sun them and still . . . smelly
I do think its a build up of smelly wee though cause it mainly his night nappies. I only wash every 3rd day or so so I think the fact that the nappies sit there for so long is doing some damage.
Can you perhaps boil them? Would that ruin the elastic? I dont know.
Other than that I would just keep sunning them. We dont get alot of sun here so I am hanging for some warmer weather to air all my nappies.