AD620 pH probe has a positive voltage offset!
Actually the 1nA bias current of the AD620 is pretty high for this application. The probe can be 100-1000M\$\Omega\$ output resistance. With your gain of 12.2 the output-referred offset with a 200M probe resistance would be about what you are seeing with a 1nA bias current. 

Normally we would use a low input bias current amplifier for this application, pA or tens of fA not nA. The bias current will not be stable with temperature and I believe the source resistance will vary with probe conditions so you really don't want to just blindly try to null it out. 

Select a more appropriate op-amp (hint: you do not need or want an instrumentation amplifier in this application).