hi,
Have you checked that as one SG is in tension the other is in compression.?
Also check that the output, in say compression drives the OPA output in a positive voltage direction.
As you are using a single voltage supply it will not be possible to drive the OPA output below 0V.
Typically with 0mV input the OPA output will be at approx +100mV or so.
E
EDIT:
Looking at your latest diagram, what is the resistance value of the strain gauges.?
Also I see no provision for nulling out any bridge zero imbalance.?
If the 120R's are not precisely matched you may have a bridge signal which is trying to drive the OPA below 0V.