Sql insert statement sometimes working other times not working

Assuming you don't have access to the mysql general query log if you're on a shared host and can't modify the database's my.cnf file, you might draft up a function along the lines of:

function dbInsertQuery($query, $conn) {
     $logFile = "/path/to/file.txt";
     $isProd  = FALSE;

     // run your security checks on the query here

     $mysql_query($query, $db_connection) or trigger_error(mysql_error());

     if (!$isProd && is_writable($logFile)) {
       if (!handle = fopen($logFile, 'a')) {
         echo "cannot open dbInsertQuery's log file: " . $logFile;
         exit;
       } 

       $logLine = date(DATE_RFC822) . " - " . $query;

       if (fwrite($handle, $logLine) === FALSE) {
         echo "cannot write to dbInsertQuery's log file: " . $logFile;
         exit;
       }

       fclose($handle);
     }

   }

This will allow you to log all your queries to a log file. I set in a $isProd variable for you to toggle for development, and if it is set to the a value of TRUE, it won't log to the file, and will skip the who fopen process entirely. You may need to adjust it, and could even get it to run every query and not just inserts if you wanted.

It could be the values you are dynamically adding to the query statement. You should escape the values to make sure they do not break your query.

In PHP you can use the mysql_escape_string() function. Documentation here: http://us3.php.net/mysql_escape_string

Also if you are running the query from a browser, you can normally see the error if you do something like this:

$query = mysql_query("QUERY", $db_connection) or trigger_error(mysql_error());