Sortd relies on Google IMAP to retrieve the Inbox. Unfortunately the Google IMAP service is quite inconsistent with regard to how long an action takes (such as fetching email) and can vary from day to day even for the same mailbox. When the Google IMAP service takes very long it is possible that Sortd will time out waiting for Google IMAP to respond.
We are very aware of this issue and actively working on finding solutions to make Sortd's Inbox a consistent, fast and reliable experience.
In the meantime, you can try refreshing the Inbox (refresh icon next to the Inbox label), or actually logging out of Sortd and logging back in (this resets the connection to Google IMAP).
[UPDATE 9 Dec 2015]
Recent versions of Sortd have started to move away from IMAP and many features no longer require it, unfortunately we still rely on IMAP to get notified when new email arrives. Sortd will now also poll every 2 minutes to see if new email has arrived so even if IMAP fails, Sortd should still show new incoming email, no later than 2 minutes delayed. Version 0.20.11 includes a fix which could have caused the 2 minute poll to fail, so hopefully that issue should be fixed now.
Note the 'show unread' filter is a filter on existing email and will not show new incoming email when enabled (yet).