Motivating unmotivated able students

I've just started my first ECT post this January amidst a school with longer term issues (new head, Ofsted looming with uncertain results) and short term ones (about half of the department went off sick at the same time when I started, so I was running semi-blind for days).

Out of all the issues, one I find the hardest is the student who when they do work is clearly capable, but will frequently not do work, or hates contributing in class when the work in their book is clearly to a high standard. My ECT mentor talks a lot about working out what motivates students, and I am struggling to fully understand how to motivate these students. Sometimes it is clearly fear (such as the ones who will copy extraneous information off the board instead of writing anything that hasn't been pre-written, even when I try scaffolding (though that is also a work in progress) even when they can answer questions verbally). However, sometimes it is either laziness or a dislike for speaking in general. How would you begin to motivate these students beyond issuing a hundred lack of engagement points.