How to recruit participants for fMRI study who do not fall asleep during experiment?
Based on my own experience as a student, the problem is probably not that your participants are lazy... the problem is that, like many university students, they're severely sleep-deprived.

While a student, I once agreed to participate in a friend's (non-MRI) study, which involved listening to sounds and clicking buttons in response. To my embarrassment, despite being enthusiastic about participating, I kept drifting off briefly during the study. I managed to complete it, but I'm sure my results were affected. I'm pretty sure that if I had ever tried to participate in an MRI study, I would have fallen completely asleep.

I don't have experience running MRI studies so I'm not really sure what to suggest to solve this problem, other than to try recruiting from less sleep-deprived populations, or redesigning the study tasks to be more engaging.