Project+ Vouchers Gone Out - Inflate my Ego

I don't know about you, but i often have a hard time getting students to fill out the end-of-day questionnaire.

From experience, I'm not alone in this: FireBrand regularly ping all their trainers that feedback responses are too low and SANS really harp on their trainers and their "facilitators" (teacher's assistant) to get >90% daily feedback.

I explain why giving feedback is important (it helps me improve, which lets students get better value). I add a quiz on the day's topic, so students can test themselves. But in the end, my response rates hover between 20% and 50%. I really don't want to guilt-trip students, so I'd love to hear more suggestions.
On Netacad (Cisco) if it's not built in then most instructors just add it in as an ungraded but required activity so they don't get a pass for the course without filling it out. Passing gives them a significant voucher discount so assuming they planning to take the cert exam, it helps get some feedback.