Beamer/Lyx bullets

[UD]

I am using Lyx (2.2.1) to create a presentation with the Copenhagen theme.  The bullets appear identical at all levels, and when I change them using the Document/Settings/Latex Preamble/Bullets dialog, I see no change in the appearance of the bullets at the various levels (Section, subsection, etc.).  I would like the second level bullets to appear differently than the top level, but I seem unable to do that.  I tried it also with the Berkeley theme, with the same disappointing results. Can anyone help? Thanks, -- Ehud Kaplan,