There are several potential reasons why your Apache Server Side Include (SSI) might not be working. Here are some things you can check to troubleshoot the issue:
Make sure that SSI is enabled in your Apache configuration. In the Apache configuration file (usually named "httpd.conf"), check that the "mod_include" module is loaded and that the "Includes" directive is set to "On" for the relevant directory or directories.
Check that the file you are trying to include has the correct file extension. Apache will only parse files with certain extensions for SSI directives. By default, these extensions are "shtml", "stm", and "shtm", but you can change this list by modifying the "AddType" directive in the Apache configuration file.
Make sure that you are using the correct SSI directive syntax. SSI directives must be placed within HTML comments, and they should take the form of an HTML element with a "virtual" attribute that specifies the path to the file to be included. For example:
<!--#include virtual="/path/to/included/file.html" -->
Check the permissions on the file you are trying to include. The file must be readable by the Apache process in order for the SSI directive to work.
Check the error logs. If there is an error processing the SSI directive, it may be logged in the Apache error logs. Checking the error logs can help you identify any issues that might be preventing the SSI from working correctly.
If you have checked all of these things and are still having trouble getting your SSI to work, it may be helpful to review the Apache documentation on SSI or seek additional help from online resources or forums.