Basically non-OP users that try to activate a command block with a button or any kind of redstone won't work unless that button or whatever is activated by an OP.
Also comparators don't output anything when the command block "outputs" a signal.
Comments 3
Is this still an issue in the current Minecraft Snapshot 15w45a or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
could it be spawn-protection rule? though this didn't happen before