Responsiv Menu with Submenus

Hi
When i have a Menu like this:

Home
Vereine
- Verein A
- Verein B
-- Article 1 Verein B
-- Article 2 Verein B
- Verein C
Links

and i click in mobile menu on "Vereine", then the submenu with "Verein A", "Verein B" and "Verein C" will open. Then, when i click in the submenu on "Verein B", the next submenu with "Article 1 Verein B" and "Article 2 Verein B" will expand.
But: that's all - the two submenu for the article 1 and 2 will open. the article behind "Verein B" will not show.
Can i solve this?

See on the site kirchgemeinde-wangen.ch (for example Menu "Vereine" and then the submenu "Gallusverein".

Thank's for your help