Sometimes the testfor command will stop working when executed by a player. It will not be able to find some or all of the entities it should, as if they weren't there. The most common example seems to be @e not finding the player while @a still does. Relogging will fix the issue.

I have found some very inconsistent ways to replicate it, but it's world specific. Using the exact same commands in a different world doesn't cause it to happen. In the attached world, simply play around with various testfor commands and trigger the command block chain a couple times. The bug will occur eventually. I have done a lot of testing and I have no idea why this exact world or command block configuration triggers the problem.