Did you have a gait analysis before the first purchase? It is possbile that the pronation she saw, if she saw it, was being caused by another, temporary, problem - e.g. a past injury or stiffness somewhere else, or an unevenness in your arm swing (have you been running carrying a bag or water), or possibly even not being used to running on a treadmill and being a bit off-balance.
In the past, I've always been diagnosed as neutral, but I've got a repetative left shoulder injury that causes my right foot to occassionally and suddenly pronate quite dramatically. I'm now wearing universally posted shoes, which are designed for everyone, whether they are neutral, over or under pronators, and I haven't had any problems with them.