NEL reporting

I was trying to collect NEL (https://www.w3.org/TR/network-error-logging/) but I don’t see them being generated. Normally Chrome would install a NEL policy upon receiving an HTTP header that instructs it to POST json reports to an endpoint determined in the header value, but that last part isn’t happening (looking at the events trace, I don’t see any attempts to POST reports). Any ideas what could be preventing that?