What are some reasons a proxy server doesn't work?

A proxy server may not work due to network configuration issues, such as incorrect settings in the client or firewall blocking the connection. It can also fail if the proxy server is down or overloaded, leading to timeouts or errors. Additionally, if the proxy is configured to filter content, it might block certain requests, preventing access to specific websites or services. Lastly, changes in the internet protocols or updates in security settings can also disrupt the functionality of a proxy server.