Reason for being air bound?

Have a friend who has a steam boiler with a loop of hot water baseboard piped off of it for the second floor that was recently renovated.

He maintains the boiler as best he can and shut the boiler down, isolated the baseboard loop and drained and flushed the near boiler piping. Not sure if he ever added cleaner or not but when he started the system back up the baseboard loop was air bound - couldn't get any circulation. What would cause this and what should / could be done to eliminate the issue?