It’s good that you found and solved the problem Adrian.
I also subscribe Jonathan’s counter rotation of the inner pin against the coax itself theory.
This type of BNC connectors I recently bought in Pamplona might be easier to connect, diagnose in case of fault and repair than the crimped ones you used.
However, I use a total of 5 PL-259 in my SOTA kit:
the one connected to the back of my FT-817,
the one connected to the transmitter SO-239 connector of my MFJ Antenna Tuner,
the one connected to the antenna SO.239 connector of my MFJ AT
These 3 are never moved nor disconnected as they are inside the wooden box containing the preconnected rigs.
See:
One short length of RG58 comes out of the box with another PL-259 permanently connected to a “barrel” SO-239+SO.239 adaptor and covered with insulating tape. You can see it on the top left picture of my 23/05/2015 post.
All I have to do when activating SOTA is connecting one single PL-259 from the antenna I’m using that day to the barrel adaptor.
The only PL-259 I use to connect and disconnect to the barrel has needed repair in both cases, the one I use with the mobile whip antennaes (on a RG213 cable) and the other one I use with the 7m long GP (on a RG58 cable).
Best 73 de Guru