This problems happens to me often: our Phabricator server is starting to have a lot of users so it's not super fast and when moving a task between workboard column, it can take 1 to 2 seconds for the modification to be done. This means if you drag a task then soon afterwards attempt to drag another one, the UI will not respond and there's no intuitive feedback. You will need to wait for the first task to be done moving (which as I finally noticed recently is reflected by its dimmed state), before moving the other one.