This is usually caused by a PHP fatal error, check the page source and PHP logs.

The most common fatal error is an out of memory error. Check the Statistics->Optimization page and see how much memory is currently assigned to PHP and how much the overview is using.

If it is a memory issue you have two choices:

• Increase PHP’s memory allocation
• Delete some of your historical data.

See http://php.net/manual/en/ini.core.php#ini.memory-limitp for information about PHP’s memory limit.

To remove historical data you can use the Statistics -> Optimization -> Purging -> Purge records older than.