A source that stopped working
A feed or a connection that has not synced. Your connections screen says since when.
What Carqo does
A source that fails repeatedly stops being retried automatically, so a broken feed cannot be read as current by being polled anyway. Your connections screen shows the last successful read and how many attempts have failed since.
What to check first
A changed password, an expired credential, or a feed address that moved are the usual causes. If nothing on your side has changed, open a case and name the source.