NTC Project (Wheatstone Bridge + Instrumentation Amplifier) - Stuck!
In your above example, you can clearly see that U2A is saturated. You should stay well away from the power supply rails (several volts is usually enough). 

With instrumentation amplifiers it is not enough that the inputs are within common mode range and the output is not saturated, you also have to ensure that the internal nodes do not saturate. The cause of the saturation is your large common mode voltage (almost 12V) relative to supply voltage and the presence of gain in the input stage. You are not the first to be bitten by this. 

Maybe the easiest fix is to remove R8 and increase R4 &amp; R6 to get your desired output voltage. 

By the way, your feedback resistors are a bit on the low side, you can increase them by 10:1 and avoid loading the outputs so much with little effect on accuracy.

And on a more advanced topic, which you may have already considered, but I'll put it out here anyway, you  have a LOT of current going through your NTC thermistor. The self-heating may be more significant than you would like. You can alter the bridge values or reduce the excitation voltage and add more gain back later. In the situation shown the power dissipation is about 10.5mW, which could represent a self-heating error of several degrees C or more (assuming a small leaded thermistor in air). Sensor makers are by times prone to quoting figures under favorable circumstances such as in briskly moving liquid.