Where are the miles actually coming from? I'm not sure if this answers your question, but if you're signing into another account (that isn't yours) to book travel using miles, you should leave the box on the award reservation page labeled "I am one of these passengers" unchecked. You can click the link next to it labeled "Why is this important?" for more details.

If the account you're using is one belonging to a Medallion and they are not traveling with you, then you're not eligible for the expanded inventory for domestic coach award travel.