Streaming RTMP via nginx-rtmp-module requires several attempts to connect when viewing the stream

Question

I'm using OBS to stream to an nginx-based RTMP server using the excellent nginx-rtmp-module.

Viewers are connecting to view the stream via the MPC-HC player and the normal URL rtmp://example.com:1935/stream.

The stream works flawlessly once viewers are connected, but the problem is it's taking them many attempts (5-10) to actually connect. There is no discernible error message except "Failed to open stream" from MPC-HC. Server-side, the required ports are opened in the firewall and as I said once viewers are connected there is no problem; the stream plays brilliantly.

Why might this be happening and how can I debug it?

Strangely, the stream does not seem to play in VLC Media Player at all (or maybe I just haven't made enough re-attempts!)