If the DVR is programmed to record on a time-schedule basis, verify that the event occurred during the scheduled record time. If the DVR is programmed to record based upon alarm input only, then check to see that the alarm input settings were properly configured and that the alarm input was indeed activated.
If the DVR is programmed to record based upon motion only, then check to see that the motion detection settings were properly configured. These settings may include a number of factors such as amount of scene change, masking blocks, and sensitivity. In order for motion to activate the DVR, the motion must exceed the settings.