Repeat type Command Blocks suddenly stop working

I am sorry for the giant lack of information but I can't narrow down any cause for this. I was playing around with command blocks, making a few TestFor things, and then the Repeat command block in charge of one of the Testfor chains just suddenly stops. If I replace the command block with a new one and put the same command, it still doesn't do anything, even if the conditions for it to output something are met.
Sometimes reloading the map works, and sometimes it just stays in nothing.
I would give more information, but the truth is I don't have any more.