Unfortunately, the Revolutionary Guard in Iran are almost a law unto themselves. If I recall correctly, she was released awhile back, and the Revolutionary Guard promptly re-arrested her, and the judge was obliged to give her a further sentence (it does not pay to cross the Revolutionary Guard).
While undoubtedly her release was tied to the payment of the debt, it was never as simple as that. I recall an interview that a previous foreign secretary gave awhile back that explained the situation in some detail. As I recall, the British Government weren't convinced that paying the debt would actually lead to her release (remember that one of the other British hostages released at the same time as her, was promptly detained again). Secondly, after paying for one of their hostages to be released, the Americans promptly announced that any organisation or country doing something similar would be sanctioned. Which meant no bank or third-country was prepared to facilitate a payment. There were several times when it looked like a deal could be done, only for something to scupper it at the last moment (such as a change of regime in Iran or America).
Undoubtedly some foreign secretaries had other priorities, and could have done more. However, it is doubtful that Boris Johnson's bungled intervention did any harm - if he hadn't of provided them with an excuse to keep her in prison, they would have found some other excuse. After all, her only real "crime" was to have once briefly worked for the BBC.
I did find her interview ungrateful. I can understand her feeling bitter about being a pawn caught in the middle. However, it wasn't the British who imprisoned her, and I heard little criticism from her of those who were responsible. But perhaps she hopes to see her family again? In which case, better not say anything that might upset the Iranians.