Some meessages do not get to Thunderbird only to Outlook Express

[earwax]

I use Thunderbird as my default email client and Plusnet as my ISP.
In Plusnet I have several mailboxes set up for the various accounts I have, and these have separate folders in Thunderbird. I am a member of Yahoo Groups for which I have two identities. I have set up Yahoo so that posts made to the groups I subscribe to are advised to me by email. For one identity I use one Plusnet account (A) and for the other another account (B). My problem is that messages for (A) get through to Thunderbird whilst those for (B) do not and can only be accessed in Outlook Express, despite all my Plusnet accounts being set up the dame way in Thunderbird.

Having tried a number of forums dealing with computer matters I am desperate to find a solution to his. Has anyone had a similar experience and how was it solved? I hope that someone might have the answer.