The Care Quality Commission (CQC) rates the Royal Cornwall Hospital as "requires improvement".
Chief Inspector Professor Sir Mike Richards said it was "disappointing" that "not enough progress" had been made its the 2014 report.
Hospital bosses insist changes have been made and care is getting better.
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The other Cornwall NHS hospitals, St Michael's in Hayle and West Cornwall in Penzance, were rated "good".
The CQC's main concerns about the Royal Cornwall Hospital included:
In recent months the trust has been ranked one of the worst in the country for waits in A&E, bed blocking and staff being able to report mistakes.
Agency spend is exceptionally high; the hospital spent Â£1.43m on agency workers in March.
The Royal Cornwall Hospital also has a higher death rate than would be expected so new chief executive Kathy Byrne has some major issues to deal with.
Prof Richards said: "If the trust is going to pull itself round, it will need a combined determination of the staff, the leadership team and the commissioners."
Kathy Byrne, the RCHT's chief executive said: "We don't feel it is a disappointing report.
"I fully accept the CQC's view that in some important areas we have not made enough progress.
"We have acted on all the CQC's "must do" recommendations and are seeing better results for patients across the board."
Cornwall's main hospital has been told it must improve, two years after it was given the same instruction by inspectors.