Depending on the type of report you are trying to generate there could be a number of reasons why your report is failing.
What type of report is it?
First you want to determine what type of report it is? This is important, because Pipe Monitoring sensors MUST use the Pipe Monitoring report.
What's the difference?
Pipe Monitoring reports can handle larger volumes of sensors, because the data this report focuses on is more specific: Was there water movement or have my taps reached the temperature thresholds?
Generating a General Event Report gives you ALL the sensor events, which means every time a sensor detected anything it is documented row by row. This can be a lot to comb through when all you want to know is "Were my taps flushed today?"
Alert History reports are a list of all the Alerts that have been triggered in the chosen time frame.
General Event Reports
If your general events report is failing it may be due to the sensor limit.
The sensor limit for reports is based on the amount of days you select. The algorithm calculates the sensor count (number of sensors) x the estimated daily events x the number of days as long as they are less than 800,000. The above algorithm is true if the estimated daily events are either 2,880 for temperature sensors, pipe monitoring sensors, average temperature sensors, desk occupancy sensors, indoor air quality and electricity sensors. All other sensors use 576 as their daily event value.
For example, if you select 5 pipe monitoring sensors, 1 humidity sensor and 3 air quality sensors for a report period of 30 days the "estimated daily events" will be 2,880 x 5 (for the pipe monitoring sensors) + 576 (for the humidity sensor) + 576 x 5 (for the air quality sensors). Or 17,856 daily events for the selected sensors. 17,856 x 30 (days) = 535,680, which is less than the limit of 800k.
Pipe Monitoring Report
For Pipe Monitoring reports see this guide for Troubleshooting the Pipe Monitoring Report.