There are some reasons for the sticky row not to work, could you check those out?
- Is inside a page layout, or inside another table.
- Has no header row or there are cells in the top row that aren't marked as headers.
- Has a header column, instead of a header row, and scrolls horizontally.
- Contains another table, that has its own header row.
More about tables in this link to Atlassian's documentation: https://confluence.atlassian.com/doc/tables-136463.html