When I had a horse that was shy I would usually have them in a dry lot for ease of working with them. I would go out there and not pay any "attention" to them and go about my business, like picking up manure, tossing rocks out, brushing the other horses, etc. A lot of times curiosity would get them to come up to me, I'd give a quick pet and walk away. Some would be like hey, wait come back here, where you going, and others would not care. I would also bring a lawn chair or bucket out there and sit and read a book/magazine with a few treats in my lap, eventually they would be curious enough to approach me without me needing to chase them down and turn it into a bad experience.
Something else to try out in addition to what others have suggested.