I recently got the 1-lb flexiforce sensor. When I wire it up to my arduino uno breadboard with a 100KOhm resistor and just let it sit flat on the table without touching it, it takes several minutes of wildly varying readings (by hundreds) for the sensor reading to eventually reach close to zero. When I do try to touch it, even if I touch it as light as I can with my fingertip, each consecutive touch induces even more hysteresis and the thing basically becomes useless because with each press everything drifts upward and decay time worsens even more. I have several other FSRs I’ve been trying from other companies and none of them have this issue.
You probably have a floating input. Check all your connections for continuity, especially the ground connection.
Can you post a picture showing how you have the sensor connected to your Arduino?