Why isn't my telephone socket working fully after installation?
So this ADSL/VDSL ?

The outlet you're using is an RJ11 ?

By the sounds of it you have too many wires coming for most mechs, Krone can take up to 4 due to double punching and 2 terminals but most can't.

Test your speed at the ISP demarcation point. Just because the 'main' socket has the most cables going to it doesn't actually mean it is the closest socket to your lead in cable. It is quite possible one of these other 'extension' sockets is actually the first outlet and you potentially have cable issues from to the 'main'.