Does anyone know what causes a field to be stuck at the bottom of a form? I have 3 fields in the main section on a sales order form which can't be moved up

Have you assigned field group to it.

no field groups in the form on that tab, but it behaves the same way as if it was in the wrong field group

Try making a new form. I've been having problems lately where starting from a standard form helps. Pain in the butt.