If you have a PurpleAir Touch or PurpleAir Zen, you can double tap the sensor to change the LED brightness.
If you have a PurpleAir Classic device, it’s possible that the internal power cable could be loose from what you’ve described.
If neither of the above are the cause of the issue, please send an email to contact@purpleair.com. We want to ensure your sensor is working properly, and not just barely.