Lunar Lights Fading?

So I had 3 sets of lunar lights on my tank and decided they where looking pretty dim and I wanted to rearrange them a bit differently anyway so I added 2 new ones, the 2 newer ones are about 50% more bright as my older leds, all the same brand  current lunar moonlights...

is it normal for leds to lose brightness over time or what do you think ? there is no salt build up or anything on the ones I've had for a while and by while we are talking less than 2 years...