Menus that DO not expand

Hi all,
I'm using the standard bluemarine theme.
I'm having an issue with menus that DO not expand.

steps are:
1) create a new MENU ITEM M1 as a primary links. Creating this menu I do not check the "Expanded" check box
2) create a new MENU ITEM M2 that has M1 as parent

Now M1 appears as a primary link, BUT when I click on it the M2 does not appear!

any help?

f.