Emails not getting through to Thunderbird In Boxes

[#1]

We use various PN email accounts that are directed to their own individual in boxes. Sometime in the last fortmight I decided to check PN Webmail and found that a dozen emails for one of the accounts were sitting there, despite my habit of instructing Thunderbird to down load the account's emails.
Today I logged on to another account, as I had done very recently, and 39 emails appeared, which were dated from early April.
Other accounts, identically set up, have not been affected.
Can anyone explain and advise?

[#2]

Quote from: lucerne

Can anyone explain and advise?

Sounds to me like it's probably due to one of the following:

• You don't have the account set up in Thunderbird or the account settings have somehow become corrupt (deleting/recreating the account in Thunderbird should fix that).
  
• Those two accounts are configured in Thunderbird to 'leave a copy of the message on the server' - are they messages that have already been downloaded by Thunderbird? (I'm guessing not).
  
• Those two accounts are configured in Thunderbird to poll the servers using IMAP instead of POP3 - again, are they messages that have already been downloaded by Thunderbird?
  
• The messages in Webmail are not in the inbox and instead are in a custom folder, if that's the case then Thunderbird will not download them over POP3.