The visibility and availability of external SMS/MMS messages depend on SMS phone numbers selected in your account. You may either not see any messages or some of your conversations may appear to be missing (note that internal chats will always show up regardless of your SMS number selection.)
When this happens ensure that at least one of your SMS lines is active in your account.
If you switch off all the numbers you will not see any prior messages. If only one number is active you will only see the conversations previously received on it.
You can also activate the default SMS number (which is selected in the Phone settings) simply by sending a new message with it.
If a line is inactive you are still receiving messages for it. These are stored on the Telebroad server and become accessible as soon as you activate the line.