Which patient circuit
are you using?
Can we assume that you've selected the correct Circuit Type at the Setup menu ... and that you've got the Exhalation Valve Line connected to the machine?
You mention "flow measurement" ... so your patient circuit includes a Flow Sensor, then?
In this case, it would be helpful if you could take a picture of your set up and attach the image to your reply.
By the way, there are some nice YouTube videos on the Trilogy (various models).