If you have a command block of type Impulse or Chain, with a saved command, and then you change its type to Repeating, it no longer executes its command.

To reproduce:

1) Give yourself a command block /give @p minecraft:command_block
2) Place it down, enter any command, and press Done.
3) Go back into the block's GUI, change its type to Repeat and Active Without Redstone, and press Done. The command no longer executes, and won't do so until broken and replaced with a repeating command block from the start.