Wheatstone bridge sensitivity and accuracy
Cool project!

I suspect that your issue may be mechanical, not electrical.  It seems that the sensor's geometry is such that it doesn't work well when just sitting on a flat surface.  You're supposed to mount the sensor by the edges of the "E"-shaped bracket, and then apply weight to center "bump".

You could jut put a couple of popsicle sticks under the edges, to try it out.

Good luck :)