Why doesn't Thunderbird get all my mail when i click on the "Get Messages" Cloud icon top left corner under file menu?

Hello, I just switched to Thunderbird from Outlook and so far it seems nice but one problem when I click on the get messages cloud icon on the top left of the screen I receive no incoming messages. Now if I go to one particular email account (I have 8 email accounts) and right click and left click on get messages, new messages come in. So my questions is, why doesn't all my mail come in when I click on the get messages icon? Am i misinterpreting this? Thank you!