I have seen this happen when importing models. It could also happen if you create a model from scratch in MED. I believe it is caused by "groups." If you have vertices in different groups, then you can only select them when you are focused on that group. However, groups are only based on faces, not individual vertices, as far as I have been able to understand. Once a group is assigned, I'm not sure it can be removed, so this has been a strange problem for me in the past. One thing I've been able to do is to export to .obj, then re-import the model. The groups are all removed in that way, and you should be able to work on it again.