I lot of people, myself included, feel that the writers have bungled or given too little development to Princess Twilight beyond episodes 1&2. However, I have theory on why we've only seen "winged twilight" and not "Princess Twilight".

 

My theory is that we'll only see princess Twilight when a third party is present. If you look at the episodes 3-7 we've not seen Twilight interact with anybody but her friends, except far Daring Do. Personally, I think Daring should have shown some sort of surprise having a princess before her, but given her character, and that her world seems far removed from Equestria maybe not. I really think the writers are keeping the relationship between the Mane 5 and Twilight exactly the same, which explains why we had that weird scene in the Everyfree when Twilight is sent home. It at once codified that they need Twilight and should not treat her differently.

 

Fluttershy: "Equestria needs its princess but we need our friend."

 

If my theory is correct, and I hope to Celestia

it is, we should see the return of Princess Twilight in the next episode-Rarity takes Manehatten. If we don't it means the writers have dropped the crown.

Source: Season 4 Twilight, a theory