Op-amp subtractor: problems supplying a fixed base voltage to be subtracted
When you hook up the voltage divider to the amplifier, the divider has to supply some current through R1. Because the R values in the divider are so high, this makes a big voltage change at "A".

From the point of view of the input to the amplifier at "A", the op-amp's noninverting input (pin 2) is held at a fixed voltage by the feedback action of the circuit. It's a lot like a virtual ground except not at 0 V.

You can solve this by using much lower values in the voltage divider (say, less than 1/10 of the R1 value, depending how accurate you want the voltage to be) or by adding an additional voltage follower buffer between the divider circuit and the op-amp circuit.