If the OP is shooting shallow DoF portraits, and the change in distance from the frame to the eye is an issue, then by definition he must be on a tight head shot and a single AF point will be smaller than the glasses frames. Use it carefully, shouldn't be a problem - that's always a tricky technique, but it's not a technical issue.
However, without seeing a picture, I still suspect the problem is most likely the subject moving after focusing and/or the camera moving. Or the usual focus-recompose thing.
A polariser won't help at all, unless the angles are very unusual (30-40 degrees to the surface) and polarisers have to be used carefully with portraits as they can remove the sheen from the skin which, apart from giving the face shape, makes people look like putty.
If none of that works, then it's manual focusing - and that's difficult at the best of times.