Re-enforcement may end the problem. Transfer the volunteer to a new position. You may, on the basis on a short interview, have mix-read their skills or inclinations. They may simply not be getting along with the staff or other volunteers with whom they are working. Send them back for a second education.
Some people take longer than others to learn new techniques.
If the problem is lack of knowledge rather than lack of motivation, then work to provide the knowledge. If a long-time volunteer has started to malfunction, they may just need a rest. This is particularly true with volunteers who have intense jobs, such as one-time work with troubled clients. The volunteer may not realize or admit that they're burned out. Give them a sabbatical and let them re-charge.