Sometimes the report is blank and sometimes it has data

Solution 1: you selected parameters that resulted in no data being pulled. For example, you may have mixed up the start and end dates, so your end date is before your start date.

Solution 2: if it's not because the report is correctly displaying no data, it could be due to the report having a long runtime and incorrectly handling a GETDATE() function, or similar. In other words, you get the current date and time at the start of the report and by the time the report is finished loading, the current date and time has changed, so it gets confused and wants to correct itself.