In-amp output lower than expected
You're applying ~9 V to IN+, but only using +5 V supply. That's going to cause problems. Try changing V7 to a 5 V supply.

The input voltage operating range spec is -Vs + 1.2 V to Vs - 1.2 V, so really you should make sure the input voltages are between 1.2 and 3.8 V.