Yangminglin,
You need to present a proper post here.
1)How exactly is the pressure being used to determine flow? (DP across an orifice plate? or DP across a pump and then working flow from the pump curve?
2) what exactly is the issue? Pressure readings fluctuating or pressure readings going to a max figure then no more? Very unclear.
3) What investigations / conversations with operators, designers and vendors have you had?
4) what exactly is your question or request for information?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.