Hello, welcome to the forums.
You may need to use the option 'Clearing the Northern Road'. Perhaps Rome is having trouble crushing Demetrios and is not yet ready to declare war on you.
Clearing the Northern Road - If Carthage controls Saguntum and Rome has not yet achieved victory over Pharos, Carthage will have the option to clear the northern Road for 5EP. This will gain Carthage 1NM and allow forces to enter the coastal regions north of Saguntum toward Rome.