Fixed Navigation on Nueva Template (not working on all pages)

I am using this code on the Nueva template:

.Header--bottom.Header--overlay { box-sizing: border-box; position: fixed; top: 0; left: 0; background-color:white; z-index: 1000; width: 100%; }

It fixes the navigation on three of my six pages. All of them are index sections with pages within but for some reason it only freezes the nav on half the pages. Any idea why this would happen?

I tried removing the overlay which was suggested in another forum question but all that did was reverse the problem.

you should share url to check @sm273007