You might try disabling the Session IP Check under Setup > General Settings > Security tab, that would help if your IP address changes frequently.
If your IP is static then are you able to stay logged in with "Remember Me" ticked? If so it's probably a PHP sessions misconfiguration on your server.
If when entering login details you are redirected back to the login screen without any error, this indicates an issue with PHP sessions on your server.
This can be confirmed by ticking the "Remember Me" checkbox on the login page:
If you are logged in with this option ticked, it confirmes there is a problem with PHP sessions. Common things to check for include:
- The configured session tmp path not being writeable
- The configured session tmp path being full
This is a PHP configuration level issue and therefore for shared or reseller users, please contact us!