We have run into that before as well. Make sure the fuel pressure is right and if there was anything left in the fuel line or pump, etc. then it likely clogged the new nozzel as well. After we got the fuel pressure right and put in a new nozzle and temp sensors we were good to go. Also double check that they are using the right nozzle. When we picked up the Parliament, the previous serving firm put in the wrong nozzle and had the fuel pressure way to low.

If you keep the bus for awhile, you can buy the pressure gauge and the testing box for about the same money as an annual pm.

There are other threads on POG where folks have had bad bearings that heated up and slowed down the speed of the unit but fortunately that was not our problem.