The most common reason a sticky header isn't sticking on your Replo page is because of a conflict with your theme, or another element you've added in Replo. Typically this is a CSS setting with overflow: hidden or overflow: scroll on your Shopify theme.

If you have this in your theme, you can ask your theme developer to remove it, and insert a custom HTML element with this code instead.

<style> /* whatever selector here */ { overflow: visible !important } </style>