My reports are often not including all of the data. So when I notice this happen I regenerate the report.

I've been trying to regenerate my reports throughout the day, but I keep getting the error message ' Sorry, we are unable to regenerate this report'. 

I tried re-creating a report with no filters but it still excluded data.

Are you able to explain to me why there are often problems with reports not including all data requested and also why I cannot regenerate my reports.