I think beam in one of those things where a lot of girls have some fear issues, my dd is 7 but is not at the stage where she does backwards walkovers on the beam yet, the older girls who have been in dd's group longer have started to learn them. My dd has has some fear issues doing forwards rolls on the high beam after a fall once, she is now becoming a little more confident on the beam and will do them (they have to do 5 without falling off before moving onto cartwheels on the floor beam), Dd said that the girls don't really like the beam all that much, dd said it in her third favourite thing, it used to be her fourth favourite piece of equipment.
I don't think there is much you can do, as I have found out, it is just something they usually get over in time, I often explain to dd that gymnastics is small bit talent and a whole lot of hard work and that sometimes you have to let your brain win over your body and tell your body what to do even though it might be hard work.