I have Greenkids Bamboo & love them. Not got the hemp onesJust thinking aloud here
I'm assuming as it was preloved that it had all ready had loads of washes, so up to absorbancy, so can't be that.
It may have build up so absorbancy reduced. Could try stipping.
Is insert soaked when you take nappy off. If it's not really wet then suggests a problem with fit of nappy rather than insert. If insert soaked through then either need more boosting or insert needs stipping. It could also be a power wee as hemp is slow to absorb so might need something on top to deal with initial wee . sometimes wrapping in m/f cloth is enough or adding some cotton terry (a flannel). might be worth a try before selling.
Stupid question, but is it laying flat & not bunched up.