Work with them to find the cause of the ennui - boredom, need a change of "scenery", a problematic SME, a lack of needed skills and a reluctance to admit it, or something else, and then address the identified problems. If there's no actionable "reason" for the lack of motivation, ask them if they think they need to change careers, need mentoring, or how they would appreciate encouragement. Check in weekly with this person and if nothing budges in two to four weeks, consider formal performance improvement coaching.