Are the batteries good and charged?
Are you using alkaline batteries? If so make sure they are really good quality and fully charged. The type and quality of batteries make a huge difference in flowlights:
The difference between a rechargeable NiMH and a cheap or low alkaline can be 7 hours of uninterrupted run time versus 30 minutes and rapid disappointment.
New rechargeable NiMH do NOT come charged as they discharge over time - make sure they are fully charged shortly before use.
Flowlights were designed with rechargeable NiMHs in mind - they draw a lot of power and most cheap alkalines can't keep up.
Do not use rechargeable alkalines or NiCD - rechargeable alkalines cannot handle the current draw of the flowlight and NiCDs don't have a lot of power for their size and don't last long.
Have you tried cleaning the battery contacts? Take a Q-tip and some rubbing alcohol if there’s any corrosion. We highly recommend cleaning and applying vaseline to all the battery contacts - a tiny bit works wonders and should last for many months. The battery contacts might have some oxidation that is interrupting the connection between the battery and the light. This is exacerbated by the vibration between the battery and the contact. The vaseline helps with that :)
This one’s silly, but it happens - Check to ensure the clear plastic battery tab is not obstructing contact between the battery and battery contact.
Check for cracks
Check the area between the button and the battery door - remove the battery door out and pry the shell apart - if there is a crack, that's considered physical damage. We replace physically damaged flowlight-FS for half-price ($7.50 per G1/G2 flowlight OR $12.50 per flowlight-FS).