The anonymously authored Augustan History, compiled in the 3rd Century, provides further detail, noting that when Hadrian became emperor, "the Britons could not be kept under Roman control".
The British problem was of deep concern to Roman central government. Thanks to a tombstone recovered from Ferentinum in Italy, we know that emergency reinforcements of over 3,000 men were rushed to the island on "the British Expedition", early in Hadrian's reign. The emperor himself visited the island in AD 122, in order to "correct many faults", bringing with him a new legion, the Sixth.
The fact that they took up residence in the legionary fortress of York suggests that the "great losses" of personnel, alluded to by Fronto, had occurred within the ranks of the Ninth.