Hi Reijer,
Do you have a mouthpiece visualiser? When working on straight airstream, often it changes back down when we place the mouthpiece on our lips. With the visualiser you can blow through this and straighten it up again. If i don’t have it handy i either use my fingers or actually blow the airstream angle slightly upwards.
You should always think about the air angle and forget thinking about moving your jaw forward imho.
Good luck