Are all proxies marked with an X in the Bad Response column? If so, the problem may be caused by some incompatibility with other software running on your computer, or specific parameters of the Internet connection. Please run the Compatibility Test from the Help menu in A4Proxy and study the results. If some of the proxies don’t have an X in the Bad Response column, it is likely that you filtered them out yourself.
A4Proxy performs two functions:
It stands between your browser and the Internet and redirects all requests from the browser to the proxy servers in the database; as a result, the website doesn’t have your IP address, and has the address of the proxy instead.
A4Proxy can also check proxy servers for anonymity. In order to do so, A4Proxy makes the proxy connect to your own computer, as if it was a web server (the proxy connects to the A4Proxy software). This enables A4Proxy to analyze the anonymity of the proxy server very thoroughly, but on the other hand, if e.g. you have a firewall, the check results will be invalid, because the proxy cannot connect to your computer and A4Proxy will decide that the proxy is not functioning. However, function #1 will still work, even with a firewall. There may be reasons for the test failure, other than a firewall: your computer may not have a public IP address, or your provider may be blocking incoming requests, etc.
What can be wrong:
A firewall (installed on your computer), blocking inbound traffic to A4Proxy (during checking, A4Proxy makes proxy servers connect to your computer as if it was a web-server. If the firewall blocks inbound traffic on port selected in the Check Port option in A4Proxy (by default, 80), checking will not work. Some information on using A4Proxy with firewalls can be found here.
Note that during the checking, requests come to your computer from the “outside”, unlike ordinary browsing, when requests go from your computer to the destination servers. Incoming requests are never required for ordinary surfing and therefore they are often blocked by firewalls and sometimes by providers, and routers have to be specifically configured to forward them into the network.
Your provider may block inbound traffic on certain ports (a sort of firewall operated by the provider). Try selecting a different port in the Check Port option on the Proxy Options tab in A4Proxy. E.g., instead of 80 which is the default, select 8080. You can also enter any other number there.
Conflicts with other software. Not all conflicts with other software can be detected by A4Proxy automatically in the Compatibility test. Some firewalls may not shutdown properly and still prevent incoming messages from reaching A4Proxy even if you reconfigured them. Some other software may also cause difficulties, e.g. the Freedom software suit, which is incompatible with A4Proxy but cannot be detected automatically.