Menu drop down hides behind hero image

HI

I hope someone can help me.

When i do the pages half of the menu drop down from the navigation bar seems to hide behind a hero image that i placed.
I read is about a z value but i cannot find the solution for it.

It only happens with the subpages of the site. The problem only occurs before i scroll down while dropping down

Hopefully someone can help me out! the site is : https://www.mamadjambos.nl/producten/jam/aardbei-rabarber-jam/ thats an example of a subpage
Thanks in advance