I can tell you what I've done a number of times with a ball python: I wet a washcloth with cool to cold water and used it to rub the head over the eye toward the nostrils a number of times. The eyecap would loosen and I'd see it being pulled out from under the ocular scales, then it would come off completely.

I always did this as soon as I noticed a bad shed. If it has been longer with the retained eyecap for your retic, I would imagine that placing the snake in a wet bag for a while before the procedure would help.

Bill