In 14w33c, I noticed that my nether beacons stopped working, however if I open up the bedrock, they come on. Possibly related to the recent changes regarding interaction with stained glass and other light-permitting blocks. I see there is an OLD issue (MC-4405) regarding the need to clear a path to bedrock, which is still true, but is now even worse, as you need to also clear the bedrock itself (i.e. in creative mode).