I have had my ms light for over a year and I am seeing lower runtimes than when I bought it but that is the norm for all lights.. It is always a possibility that you have just 1 bad cell in the 4 cell battery pack and this is the problem. Due to the price, I'm sure they are using low cost 18650 cells in the battery packs..

I always top my battery off before riding, just in case I get adventurous and decide to ride for 2-3 hours.. I always have someone extra on my bars in case the battery runs down, it has only happened once on 3 hour ride, switched to the lower mode to preserve battery once the red light came on..