When you go online with the PLC, do you see an input from the pressure tranducer that isn't 0?
Start at the pressure transducer, make sure it's working correctly.
Examine the signal wire from the pressure transducer for any damage.
Connect to the PLC and see if it's getting the value from the transducer.
We have had some Wonderware HMIs lock up certain numeric displays until we rebooted the PCs. I'm not sure why it happened, but a position field locked onto a number, and even when the value changed in the PLC, the display wouldn't change. Once we reboot the PC, the problem resolved itself.