Thunderbird: Switch from POP to IMAP account
POP is an outdated protocol not suitable for checking mail from multiple devices.
If your account has initially been setup as POP, here is a link to a good article from Mozilla Thunderbird explaining how to convert your POP account to IMAP.
Note: It is important to preserve the original POP account as it has all the messages previously downloaded to your computer.
You can delete the POP account once all the messages have been transferred to your IMAP account, or keep it as archive.