Have a thorough read through the thread I've linked below Tony. I had a similar problem to you (ignore the fact I wrote that the RCD was faulty in my first post in the linked thread, I was wrong as I suspect you may be) and this thread helped me to understand why a perfectly good RCD will not trip when tested from a socket or light rather than at the RCD itself (with outgoing terminals disconnected).
It is always unlikely that the RCD is faulty, not impossible, just unlikely. I have only come across a handful of faulty RCD's in the last 10 years.
Here is the link/thread
Why isnt the RCD tripping? - https://www.electriciansforums.net/threads/why-isnt-the-rcd-tripping.176881/#gsc.tab=0