BT Versatility - analogue extensions not working

We have a BT Versatility phone system, and have been using 5 of the possible 8 extensions.

I have recently attempted to add some more extensions to the system.

This is a 4-wire hybrid system. A and B on the CCU connect to 2 and 5 in the socket to provide an analogue voice path, and C/D connect to 1/6 to talk to the digital extensions.

However, on two of the extensions (ports 5 and 8 on the CCU), there is no analogue voice path. SLT's don't work, and digital extensions cannot make or receive calls.

I've connected a socket via a very short length of cat5 to eliminate cabling issues, and the phone I am testing with works fine on other extensions - just not these two.

I've checked that these extensions are not disabled via programming.

Is it likely that there's a fault on the CCU affecting these two extensions, or is there anything else in the programming that could affect it?