An MP is calling on the government to launch an inquiry into the performance of the East Midlands Ambulance Service.
EMAS is in the second of a three-year investment programme
Harborough MP Edward Garnier says the inquiry is needed to get to "the root of the problem" at the service.
But Health Minister Rosie Winterton has turned down his request, saying local primary care trusts are responsible.
East Midlands Ambulance Service (EMAS) said: "We are now achieving, or are ahead of, the targets agreed with PCTs for category A life-threatening calls."
Mr Garnier said: "Their management systems must not be working up to speed and they may need some outside assistance - perhaps from Whitehall - to tell them how to run the show.
"Whatever it is - my constituents are not getting the service they paid for."
The EMAS is in the second year of a three-year investment programme designed to enable the PCTs to achieve national targets.
"We have made excellent progress in responding to the non life-threatening calls we receive from GPs," a statement from EMAS said.
"EMAS is encouraged by its recent improvements in performance but we recognise that we still have work to do if the people of the area are to benefit from an emergency response service that is of a consistently high quality.
"Nevertheless, the success attained so far is very welcome and shows that the investment is beginning to reap dividends."
EMAS scored two stars out of a possible three in 2004 but the rating fell to one in the latest figures.
It was found to have "significantly underachieved" when answering serious, but non life-threatening calls.