Achieving a solid sound and attack in the lower register takes work. You can’t force it. Personally, I (and many people) pivot the mouthpiece a bit in search of the best placement for lower notes. I believe all of this pivoting serves the goal of allowing the lower lip to set in the right place for best vibration.
Also, lower notes call for a more air stream to allow good resonance. Some people like to think of ‘warmer’ air such as that you would use to fog up a mirror.
Here’s a pdf of Octave Leaps 
These exercises also have a
dedicated page on TromboneZone.org