As we know, the Iceni tribe in Norfolk raised rebellion against Roman rule in ad60 whilst the Governor, Paulinus, was far away ravaging Mona (Anglesey).
He eventually cobbled together part/whole of the XIVth and XXth Legions (the IXth had been badly mauled by Boudica's ambush near Lincoln) to meet him somewhere in the midlands (?) and summoned the IInd Legion - then based in Exeter way to the south.
But the IInd- under it's third-in-command, Poenius Postumus (we assume him to be capable and brave to have achieved this level of command and trust?) failed to join him. Why? Is there something missing in our known (Roman-sourced) history of this event? Was the rebellion more widespread?
Were they being besieged at their camp by the local Dumnonii tribe, maybe having allied themselves with the Iceni- as had the Trinovantes and others?
Presumably the top two commanders of the IInd were away serving with Paulinus (?) and must have issued orders, if Paulinus hadn't?
Had Postumus heard of the disastrous fate of the IXth?
According to Tacitus’ Annals, you are right that the II Augusta’s senior commanders were with Suetonius. That left the Prefectus Castrorum, Poenius Postumus in command.
Tacitus records that Postumus disregarded Suetonius’ summons. He does not explain why, though he does note that it was ‘contrary to all military usage’.
When word of the decisive Roman victory at the Battle of Watling Street reached the II Augusta, Postumus supposedly felt that he had ‘cheated his legion out of like glory’ [note: glory such as that obtained by those legions that did fight in the battle]. Disgraced, he committed suicide.
Though no explanation for his decision has been recorded, the fact that Postumus felt so ashamed that he took his own life suggests that he COULD have answered the summons but chose not to, rather than because his legion was otherwise occupied