Sub Menu falling below Cover Images

I’ve created a sub-menu within the navigation bar using the default wordpress editor, but on any page where there is a cover image, the menu drops down below the cover image so that you can’t see/use it.

I tried getting around this on a few pages by adding the cover into the navigation bar, and it works, but I can’t do this for all of my pages.

I’ve tried looking up fixes, but so far nothing seems to work (although in fairness, I’m not sure I fully understand where to put the code).

I’m not super tech-savvy, but I’d really appreciate any help figuring this out!

Thanks so much!

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