I can't edit the original but it looks strongly like it works for one viewport height.

Is it something like this testcase? https://codepen.io/brandonmcconnell/pen/ZEZaozw/5952a63864a7d7167c788cec6e03b487

No, this works perfectly (even if I duplicate the rows). Although the structure and CSS are very similar.

So the issue is somewhere else or more complicated.

I will try and duplicate my situation in a codepen. At the moment it's in a private part of a website.

Thank you for the help.

I did pick up one thing from your codepen which has resolved the issue. It's a question of the semantic markup. If I wrap the header rows in <thead> then the sticky behaves correctly. (although removing this doesn't break the codepen). I'm happy with that as correct semantics is never a bad thing ;-)

Thanks again