$wpdb->insert is not working

Many people seems to face this error . Because the of  the wpdb syntax error, or some data missing, or format error, even some times which does not include wp-load.php. Or even some times. other functions of the file may restrict to work it properly. 

Before that you have to understand well about usage and handling of $wpdb and its functions  in wordpress. 

Its available at  http://codex(dot)wordpress(dot)org/Class_Reference/wpdb