First: A Few Words About Velocity
Velocity is most useful as an estimating tool, and as a measure of variance. However, it is often misused as a management goal or external commitment target.
Wide variance in velocity is certainly a good reason to inspect the current project, though, and look for flaws in estimation, procedural impediments, process issues, or even social dynamics. However, social dynamics is deliberately last on the list, as problems with social dynamics are usually (but not always) symptoms of process issues rather than the other way around.
Still, since the question as posed was about self-motivation, let's start there.
External vs. Internal Motivation
If a person not self-motivated, then they can possibly be motivated by external incentives, but that's still not intrinsically self-motivation. Self-motivation requires engagement with the process, and a certain amount of enlightened self-interest in the outcome of that process.
Disenfranchisement is Common
Teams aren't "teams" just because people are assigned to them. Real teaming is something that grows organically out of a successful organizational and social process.
In many organizations, teaming is imposed externally, and then management is surprised when a random assemblage of people doesn't gel together. In other cases, tasks, deadlines, and objectives are mandated from above, and then (surprise again!) the "resources" fail to take personal responsibility for commitments they haven't made themselves.
Open a Dialog
You can't really know what's wrong unless you talk to the entire group. "Lack of self-motivation" is a symptom, not a cause. I'd bet heavily that the members of the group know why the project is not meeting expectations, so ask them!
If there really is a problem with specific individuals, opening a dialog directly with them is also a good first step. Communication is critical, regardless of the project management methodology.
Inspect Your Process
This is a terrific opportunity to inspect your entire system of processes. Some things you can look at are:
- Is the project management framework a good fit for the work at hand?
- Is the project management process working as designed?
- Is the project management process working effectively?
- Do you have the right people with the right skill-sets on the project?
- Are there organizational processes that are impeding the project?
- Are the hiring/retention practices bringing in the right people, and keeping them there?
If you truly have under-performing group members, then it's a Human Resources issue rather than a Project Management one. The solution to such problems is always self-evident: don't retain poor performers, and budget a lot more if you want to recruit top talent.
However, if you don't start with the assumption that there's something wrong with the team members, but possibly something wrong with the group's process, then you have a lot of wonderful opportunities to inspect-and-adapt. Don't underestimate the power of good process (re-)engineering!