All volunteers have the right to state when they are willing to give their time. However, if there is a risk to service delivery or an impact on the organisation, you need to discuss this with the volunteer in question. But if they have other commitments outside of volunteering they may not be able to volunteer outside of certain times.
My advice would be to make your request clear and reasonable, but be understanding from their perspective if this is not possible. Unlike paid staff, who are bound by contracts, volunteers and organisations work largely on compromise.
Not being able to get up in the morning is often a symptom of depression. I usually find it helpful to sit down and talk to anyone who doesn't seem to be following the usual path and ask them whether they're disappointed with the experience and why they think they're not fitting into the normal pattern. That gives them the opportunity to express their frustrations in a positive way that allows you to dig a bit deeper into the reasons. Hope it helps
