The only thing I can think of that you give them exercise recommendations that they really enjoy and are more likely to do even when on a tight schedule. Or giving them mini-exercise assignments of no more than 10 minutes in length.
Else you have done all you can. It is ultimately their choice whether or not they follow your advice.
I have some clients that will only exercise with me. Do I like it? No. But without me, they probably would not exercise at all.