Why does the LR36R downshift to a lower mode after being used for a while and the higher mode cannot be selected?Updated 2 hours ago

There are two reasons: 

1. Intelligent Overheat Protection

The flashlight will accumulate a lot of heat when used at high output levels for extended periods. When the light reaches a temperature of 60°C or above, the light will automatically step down a few lumens to reduce the temperature. 

2. Low-voltage Warning

When the voltage level drops below the preset level, the flashlight is programmed to downshift to a lower brightness level until the lowest brightness level is reached.