You recently edited a Mapping and hit Save. Since, then Connect has been stuck for more than 5 min in the Alter Schema state and is not synchronizing data.
Being stuck in Alter Schema is usually caused by a DB query holding open locks on the table. You can check if this is the case by inspecting the output of heroku pg:locks. If there are queries that are holding locks, you can kill them using pg:kill. You will be able to identify queries generated by Connect because the application_name will have an "hc." prefix.
heroku pg:locks heroku pg:kill pid