Thunderbird flapping over new mailbox?

[#1]

I recently created an additional mailbox with a new password distinct from the existing boxes.
I can access the box on PNet's webmail using said password. However every time I try to download using Thunderbird I get this message -
Sending of password did not succeed. Mail server mail.plus.net responded: Login failed
Anyone got any ideas about this one? Thanks

[#2]

If the mailbox functions in webmail, there is no reason why this will not function in Thunderbird.
Which mailbox are you referring to?

[#3]

Thanks for the replies.
New mailbox is lukebaker.
Yes, I'm using the <username>+<mailboxname> format  in Thunderbird.
I should add that from time to time (perhaps once a week) I'm unable to download from my main box and get the following message -
An error occurred with the POP3 mail server. Mail server mail.plus.net responded:
So I'm beginning to wonder if there's something strange about my Thunderbird set-up.