Constant current source math - Using AD8276 differential Op-Amp
You've made a basic error - you have the inverted and non-inverted inputs of your op-amp swapped. Here's what it should look like: -



Here's a good document on it. Alternatively look in the data sheet for the AD8276 - page 17 top right-hand corner shows a constant current generator using an extra transistor and I suspect your design was based on this circuit but without the extra grunt from the transistor.

(This text is here to satisfy SO's 6 character edit requirement...)