.htaccess being ignored by Apache

It seems that the .htaccess file for one of my virtual hosts is being ignored on my production server. The file is identical to working .htaccess files for other vhosts on the same server. Also, the vhost definition is identical to other working vhosts, except, of course, for lines that are specific to the respective vhosts. When the problem is occurring, index.php loads correctly for the base URL, but all rewrite directives are ignored. Also, putting garbage in .htaccess doesn't cause an error. The site runs as expected on my backup server, to which websites and Apache config files (with one exception, mods-enabled) are mirrored hourly, and to which databases are replicated. I compared mods-enabled on my backup and production servers, and found that mod_cgid was not enabled in production. Enabling it seemed to fix the problem, but only temporarily. Selectively deleting rewrites from .htaccess brings no success. I'm considering rebuilding my production server. What other troubleshooting is recommended? Are there any known fixes?