Small is a funny thing. You get used to fragrances after a while and they don't smell as strong anymore. That's why someone else's washing detergent can stink but yours doesn't seem to.
Perhaps the seller didn't notice? Could the smell perhaps be a mixture of their detergent and vinegar or similar?
I had this problem once and several washes with essential oils and a day on the line in between did nothing to shift if, so eventually had to admit defeat and send the nappy back to the seller (which was a shame as I really liked the nappy).
ETA: This was a long time ago on another forum.
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