I agree that if you try reasonable pharmacological interventions for bipolar spectrum disorders (and the Phelps site gives some good evidence-based approaches) and don't get a good amount of response, then bipolar doesn't really fit. Because most bipolar will respond to meds, but you might have to try more than one thing.
Or perhaps, more controversially, you have hit on BPD that might have some medication-responsiveness. I know this is controversial but I know there have been some studies in this area.
Rheum frequently deals with this ambiguity in dx. Sometimes you don't know exactly what it is, you try meds, and what sticks sticks. If it doesn't than

and move on to something else.