Gnus not finding groups on my server

[Matthew Vernon]

Hi,

Gnus is failing to find groups on my news server correctly; if I try and
j or U and then enter a new newsgroup (that is on my server but I've
never subscribed to), gnus tells me it can't find it. But if I use B to
visit my usual news-server, then there all the full range of groups
available...?

I'm sure I've done something obvious to confuse gnus...

I have:
(setq gnus-select-method '(nntp "MYSERVERNAME"))
(setq gnus-read-active-file t)

Gnus v5.13

Thanks,

Matthew