So, I use your theme on two of my sites.
The one site, is https://progearandsettings.com – here, the sticky nav is working perfectly.
The other site, https://scrutiny.gg/classic-wow-profession-guides/cooking-guide/, the sticky nav sort of disappears and reappears when it wants? Not really sure. I tried comparing the two and they look nearly identical although I think pgs may be running an older version of Generate Premium.
Would love any insight you may be able to offer!