If this plug-in is not working on your site, most likely it is not the plug-in's fault. You have other (greater) problems. WP-Sentinel is functioning super on mine (more than 10). Conflicts can occur--google the error message (I have not received any to date.)
Excellent plug-in; smooth operation. You receive notices of attacks at designated email address daily (can be alternate or admin address).
Go to plugin interface and see the attacks blocked and IP origin. Manual ban is great!Ability to manually override automatic 24 hr ban and increase offending IP ban to your chosen length (300 hours, 300 days or more--NICE).
Many well known international hosts don't understand WP and appear to have no desire to do so. Get a WP host who caters to WP and has the current minimum requirements (php and mysql)--many don't and don't care. Their laissez-faire attitude causes you problems with plugins and installations. Don't except lame excuses from hosts...
Generally the WP installs (code) and plugins are well-tested. WP authors respond to cracks and errors quickly making it a dependable relationship. If your site/plugin is not working, first make sure your site is a perfect (error-free) clean install and your files are up-to-date. Then you have minimum hosting requirements (many hosts are weak on updating to current versions). If you can't do that yourself--get professional help. Get professional help if you need it. Did I mention, "Get help if you need it"??