I haven't completed the Brotherhood of Steel questline yet and like you haven't done more than multiple Quartermastery and Leading By Example quests (which are infinitely generating side quests and not story missions). However, the quest "A Loose End" references people and events that don't occur until after "The Molecular Level", and "Getting Technical" follows on from "A Loose End".
To start this question you will need to speak with Lancer-Captain Kells aboard the Prydwen.
I would suggest doing "The Molecular Level" using The Brotherhood of Steel's scribes to decypher the blueprints and the quests you've mentioned will unlock after that.
You must have also completed the Brotherhood of Steel side quest "Duty or Dishonor."
The "why" is explained here in this spoiler if you want more information (hover to show):
The quest "A Loose End" allows you to convince the Captain to keep Virgil alive as an asset, assuming you've cured him. Since you get the retrovirus from inside the Institute's biosciences laboratory it wouldn't be possible to cure him before completing "The Molecular Level".