Representing an equation with a simulation of Operation amplifier adder
I notice that for the middle input, without the opamp, v=14v, and with the opamp, v=13.5v. 

TL084 outputs don't go to the rails, so with 15v supplies, the output stops short of where it should be. 

Use an opamp with a R2R output, or increase the supply voltage, or decrease your test voltages so that all internal voltages are within the linear range of your amplifiers.