@gareth_bevan thanks for asking. We realize that we need instructions and manuals for the valve extenders and more. So thanks for pointing out this common confusion point.
Without seeing your setup, it’s hard to be sure exactly what’s going on, but I think you gave some good clues, so I will try to diagnose and give an answer.
Question: are you using a removable valve core tube? If not then my upcoming diagnosis will certainly be correct.
Question 2: if you are using a removable core, did you remove the core, install the extension then reinstall the core into the extension?
If either of those answers are no, the you have the vale extension installed backwards. It will fit backwards because of common threads and you will be able to pump air into your tire but it’s not the proper installation and you won’t be able to get pressure readings on your gauge or lock your valve shut.
The fix would be to take the necessary steps such that you can answer Yes to my two questions above.
Let me know if you have and questions or need any help. Thanks!