It's not bugged. Ships automatically reduce sail in order to avoid collisions with other ships, and will not let you override their desire not to smash into other vessels. As such, when you have a line of vessels relatively close together, such as how the computer automatically deploys them at the start of a battle, ones in the middle can often start reducing their own speed in order to prevent possibly colliding with the one in front.

If you have just one ship in a group that cannot be given sail commands, usually because it thinks it's going to collide with something, then the command for the whole group becomes unavailable.