Probably you cannot move due to Zones of Control. You might notice the regions around your siege are red when you click on your force. This is because you occupy an enemy controlled region. So you must complete the siege to be able to move.
Look at the wall strength. If a one or zero, get into assault mode and take the fort.
If a two or more, you must make a breach. A siege engine will help as do Roman legions.
If you are trying to take a port, a naval fleet of the required number of ships is helpful. They will not help with assault, but they will reduced the city's supply to zero and they city might surrender due to famine.