I have noticed that after I put my laptop (10) to sleep, when it wakes up and even though I have check for email on the server every 5 minutes it wont get the mail. I have to manually "get mail" Then it is fine and goes through the 5 minute setting.
for now I just remember to get mail when I turn the computer on..
"Thunderbird Help" is the first item in the Help menu, which gets you to https://support.mozilla.org/en-US/products/thunderbird
Is there a reason you posted on Connect instead of the support site at https://support.mozilla.org/en-US/questions/new/thunderbird ? There are more people to help you at the support site, than there are here in Connect.