First check that the unit is turned on and that the sensor cable is plugged in properly. Once these obvious causes are eliminated, check to see if the voltage display on the controller is changing when the sample is contracting or when lightly touching the output tube. If the answer is yes, then the unit is detecting force and you can refer to the software FAQ (600A or 615A) if you are not detecting the force change in software.
If the amount of force being registered is much less than normal, then there could be some mechanical impedance between the transducer hook/tube and other parts of the system. If the output tube or hook has recently been replaced, then inspect inside the sensor head for possible epoxy or glue under the flexure or broken flexure. Sometimes if too much epoxy or glue is applied it can flow underneath the glass/flexure and thus the flexure becomes stuck and unable to measure force. If the flexure is broken or glue is under it then it will need to be sent back for a flexure repair. If not, inspect the output tube to ensure it is properly secured. If this is loose it will not transmit some or all of the tension put on it by the tissue preparation so it will need to be replaced.