Hadrian had to builds wall
In the 1st century Agricola wasn't allowed to finish up the conquests of Scotland and Ireland (there's a promontory n of Dublin that appears to have been a Roman presence in Ireland). Hadrian's predecessor Trajan was one of the greatest conquerors ever to hold the office of Emperor, whereas Hadrian's forte was in buggering pretty young boys, such as Antinoos. When Antinoos died, Hadrian started a cult to worship the catamite and bilt a town in Egypt, naming it after the dead 'man'. Hadrian's accession foreshadowed the later decadence of the upper echelons of Roman politics. Hadrian decided to define borders for the Empire and rely on alliances built on trade and patronage, a common practice Rome used to create buffer states on their borders. During a later (the second) expansion into what we now call Scotland, the Antonine Wall was put up. Manpower was again needed elsewhere, so the Antonine Wall was abandoned for a time, during the third century (and third expansion) into Scotland was reoccupied, and abandoned apparently for good after the locals simmered down.