Please share your site url so that I will give you exact solution
As far as I understand your case, when it comes to menu it is either floating or sticky, in your case it seems it floats as you scroll down. In that case you need to stick it to the header by editing the code. So, if you can get someone who can make changes to the code in the background, you can stick the menu to the top.
There are codes for that for instance, https://community.shopify.com/c/Shopify-Design/Sticky-Fixed-Header-and-Navigation-for-Debut-Theme/td...
Use this only if you are comfortable in coding. If you need further help, let me know. Happy to help you.
Thanks and regards,