I have seen team members fall below expectations, or burnout, or otherwise become unproductive, but then recover. In every case, their manager was a key part of the solution (and sometimes a key part of the original problem). If a person is really stuck on their current work, it can be demoralizing to have it taken away and assigned to others, but it can also be a relief. One thing that I've seen help is to let devs be more self-directed. Try to nudge them towards things that are important for the business, but focus on doing that by really convincing them of the value of that work. If they don't want to do that work, try to understand why, and consider changing plans based off of that.