Did you do the storage mode thing before you stored it? ...
See below - I got this from a Knog website regarding the front USB boomer light as I'm considering buying one.
The battery discharge feature has been integrated into the electronics to optimize battery longevity. This feature should be activated when the light will not be used for an extended period. (i.e. more than a month) To activate mode simply hold down power button for 5 seconds after a full charge cycle. The LED will be active until the battery reaches the correct discharge level to which the LED will turn off and the unit will be safe for storage.
- If unit is stored for more than 3 months, repeat battery discharge feature
- Recharge battery to full before use after storage mode.