And what is the TARGET of the task? If the target is a client of a child server, then the task instance has to replicate down to the child NS and be essentially re-deployed from there. We know that, in some instances, tasks themselves will be created on the parent, and NOT replicate prior to you (an administrator) creating a run instance of the task. DS has this issue. Some other solutions have built-in to their product a method of ensuring the task replicates at the same time as the run instance.
Note, that when you drill down into a task that is destined for a client on a child NS, even though you're on the parent, you drill down into a direct view of the child NS iteself. You can actually see that at the top of the pop-up window that you're no longer on the server you THOUGHT you were on.
Take a look at that, and see if this is what you're running into. Then simply right-click the task, replicate now, and see if the problem goes away.