It is amazing what a difference energy can make. I have a mare that's basically the equine equivalent of a golden retriever, so you can march up to her any way you like (and she usually meets you halfway, as well as responding to her name), but her two pasture mates are much shyer. Walking slowly, not making direct eye contact or looking at them directly, letting them sniff you and just being relaxed makes all the difference in the world. 

Messing with Perky (my mare) also makes them jealous and is the lazy solution to the problem if you don't feel like putting the work into approaching them. They never fail to come over and see what I'm doing. 

There's a sometimes hard-to-catch Arab gelding in the next pasture who I made friends with after a good wither scratch and a treat, and now I can't get rid of the sucker! Go into that pasture, and he follows me up and down even if I'm toting a halter and lead.