Two items to check: is settlement happiness consistently above 70-80%, and do you have enough scrap of each type? Since you are running the hardest "scrap components" setting rather than the easier "scrap categories", it may be waiting for specific amounts of scrap in particular categories to be accumulated. I don't know the exact scrap requirements for ROTC Sanctuary Level 1, but if you have any types of scrap that seem a bit low it may be worth grabbing a shipment from a vendor or trying to find another source of the material that you can donate to the city.
Alternately, change from "scrap components" to "scrap categories" in the options, toggle in/out of workshop mode so it refreshes your stockpile, and then give it an in-game day or two...you'll probably upgrade.