I've found that, with scoreboard objectives, if the objective has a dash ("-") in the name, then the score argument for command blocks will treat it as invalid, which also displays
- Start a new world and give yourself a command block.
- Type in the command say @p[score_Foo-bar=44]
- Power the command block and it should say your name.
- Click on the command block and remove the "-" so it should now be say @p[score_Foobar=44]
- Power the command block again and notice how it doesn't say your name this time.