The obvious question..
Are they setup on a dimmer?
Sometimes things like this will have some sort of dimming control, could be a knob on the on/off switch, could be a button on a remote control..
Start there and rule that possibility out..
If no dimmer can be found then start looking at voltage at the LED panel, if voltage is at or very close to battery voltage (IE "12V") and you do not have a dimmer then the issue is most likely failing LED strip and the only repair is to replace.