"Repeat header row" is set, but not repeating

Yes, header rows only repeat under specific conditions: they must be the first row(s) of a continuous table, the table must not be interrupted by manual page breaks, and rows cannot be set to “Do not allow row to break across pages” if that prevents Word from splitting the table. If these conditions are not met, the “Repeat as header row” setting will not display correctly.

https://support.microsoft.com/en-us/word/repeat-table-header-on-subsequent-pages

One more thing to check: Ensure that there are no manual page breaks added inside the table, because those breaks will split the table, which, in turn, will prevent header rows from working.

As an alternative to manual page breaks, you can control page breaks by adding "Page break before" formatting to a whole table row. 

The repeat setting only works when Word sees the content as one continuous, inline table. Disabling Allow row to break across pages does not control whether the header repeats; it only prevents an individual row from splitting between two pages.

Check the table in this order:

1. Switch to View > Print Layout. Repeated headers may not appear in Draft or Web Layout view.
2. Select the first row of the table. If the header uses several rows, select all of those rows starting from the very first row.
3. Open Table Layout and select Repeat Header Rows. You can also confirm it under Table Properties > Row > Repeat as header row at the top of each page.
4. Open Table Properties > Table and set Text wrapping to None. A floating table using Around wrapping may not repeat its header correctly.
5. Press Ctrl + Shift + 8 to display formatting marks. Check for a paragraph, manual page break or section break that divides the table into two separate tables. Remove the break so it becomes one continuous table.

Also check whether the table is nested inside another table or contains vertically merged cells across the header and body. Those layouts can interfere with repeating headers and are best avoided in an accessible template.

Finally, check File > Print. If the header appears in Print Preview but not in the editing view, the repeat setting is working and the difference is only the current Word view. If it is also missing in Print Preview, the table is most likely split, floating, or the marked header is not the first row of the table.

See Header Rows in my page on Tables.

If your table is set to be "floating" to have text wrap around the table, header rows will not function.

You can have multiple rows included. For more than one, they need to be contiguous rows at the top of the table.

Word repeats headers only when the selected rows are the table’s leading rows and the table flows continuously across pages. Select the entire first row, not just its text, then open Table Properties > Row and confirm Repeat as header row at the top of each page. Switch to Print Layout and check Print Preview, because repeated headers are not displayed in every view. Turn on formatting marks and verify there is no manual page break, paragraph, or second table between the first-page portion and its continuation. In Table Properties > Table, temporarily set Text wrapping to None; a floating table can behave differently from an inline table. Confirm the apparent header is not built with merged cells extending into non-header rows. “Allow row to break across pages” controls row splitting and does not enable header repetition.