If you are trying to generate the stats report for a selected time frame without success, the reason could be probably related to scalability.
Starting a report generation the browser could hangs for several minutes without generate the final report, or even results in a server error or a blank page or a 504 timeout server error.
In this cases probably data to elaborate in the selected time frame requires too much time and resources. You can try to generate single sub-types of reports instead of the full stats report or reduce the time frame interval to ease the process.
This is strictly related to number of visits of the site by day, and so the number of stats collected every day. On high traffic sites, the database will grow quickly and processing data for 1 week could mean analyze thousands of records to calculate final stats report. In all these cases you have to reduce the amount of data processed or increase server resources.
If you can't increase the server timeout for PHP scripts, you can reduce the default timeframe used by JRealtime Analytics to process collected data using the configuration parameter visible in the image below choosing 'By day'. Ensure to logout/login from the Joomla backend once changed this setting to apply it.
This issue may happen only when you try to generate PDF reports, elaborate and translate to PDF format a huge amount of data is even more resources and CPU expensive. The result could be a blank page that hides a server error under the hood, most probably due to memory limit exceeded or script timeout. You can enable debug mode of JRealtime Analytics and Joomla to display error details on screen during report generation.
Using a different format for reports such as Excel or CSV probably will solve the problem and it's highly recommended for all cases in which huge amounts of data must be processed.