How to solve this Bevel problem?

So I was beveling my rail here. It always doesn’t work right when I bevel both sides of it, as shown in the picture.

Any idea how to solve it?

No blend file supplied to review !! Why not ??

Ensure you do not have ngons (Faces > 4 sides)
Ensure you do not have duplicate geometry
Ensure you have no internal faces
Ensure your face normals are all consistent
Ensure you have applied any object scale (Ctrl+A)
Please ALWAYS supply a demo blend file that shows the issue

Here it is:

Blender seems to do exactly what you tell it to do…?
From the looks of your screenshot you only select the top edge (orange) before firing the bevel command, but not the adjacent (already bevelled) small edges (red):

So, how is Blender supposed to know you want to bevel them, too?

That just solved my problem for long instantly! Thanx for the help!