I have had numerous tail lights and have the same problem. I recently got a new bike with sloping top tube so there is more room for a light mounted to the seat post.
I decided to get a Bontrager Flare RT which is a really nice light
But it was still being obscured by the seat bag. My bag has a strip to mount a light to but it is angled down.
I cobbled my own clip that angles the light upward and that solved the problem.
Bontrager makes a seat stay clip which has adjustable angle so the light will be visible.