Wow - that's a huge question! I think the best way to approach this would be to break the problem down and consider the factors involved that may cause the reluctance.
Factors external to the student could be the teacher, the learning environment, the course content, the length of the sessions etc etc.
Internal factors i.e. to do with the learners, could be the students interest in the subject, their ability, their learning style (girls are different to boys for example), their learning history, what's going on in their life etc etc.
Once you have identified the main factors, you can actually start to think of some solutions that will turn the situation around.
That's how I would approach the problem anyway.....any other ideas?