Richard's hunger strike was from October to November. Liz Truss became foreign secretary on 15 September. This is from inews.co.uk's article on how her release eventually happened.
"In September last year, Ms Truss met her Iranian counterpart Hossein Amir-Abdollahian at the UN in New York. They issued only a bland public statement afterwards – but in secret they agreed to accelerate the endgame for negotiations, crucially using Oman as a neutral go-between.
Throughout October and November, British and Iranian diplomats repeatedly met in Tehran. In total, there have been more than three dozen meetings and 100 more conversations over the fate of the prisoners.
Ms Truss had the foreign minister of Oman over to her country house, Chevening, a few days before Christmas. On the face of it, this was nothing more than a friendly catch-up with a number of Gulf governments represented. But on the fringes of the main event, the two allies peeled away to discuss the prospects of a deal with Iran.
Oman’s capital, Muscat, was the venue for a final meeting between Ms Truss and Mr Amir-Abdollahian. Meanwhile Foreign Office officials had been working around the clock – one senior figure estimated they had racked up overtime equivalent to an additional 50 per cent of their normal hours – and ministers had spoken to the families of Ms Zaghari-Ratcliffe, Mr Ashoori and Morad Tahbaz more than 75 times.'
That's not to defend the failures beforehand but I don't get the whole anti-Truss thing. It seems like she did a good job. This is just one article though, and doubtless briefed by the Foreign Office. An investigation's obviously required.