A common frustration in College Ministry is dealing with students in responsibility roles not doing what they have committed to do or are assigned to do.
Obviously, there is no magic answer....but, here's some things that may help.
1. Try to be clear up front what the responsibilities and time commitments are......don't assume they know.
2. Meet periodically with them to discuss how it's going, to evaluate and help them learn from recent experiences.
3. When they do not do something they are responsible for, don't ignore it. Discuss it with them...why did they not do it, show up, etc? If it is not brought up, perhaps they get the message it doesn't matter.
4. Decide what is the lowest degree of "acceptable". It may not be done on the level you would like it done, but it is acceptable and you will live with that.
5. Degree of "acceptability" may be determined by the degree of experience of the student, their age,and the effect of their actions on others.
6. If they consistently perform below what you have decided is acceptable, it is time to have "the talk". The talk is, "you can't continue at this level of performance and stay in this role". Part of our role is help students understand responsibility, if we don't, we have failed them as a College Minister.
By the way, word will get around, that you are willing to have "the talk". You might be surprised the positive effect on other students. But, you must be fair, honest and straight with students. And, you must always be willing to give a second chance. But, we are wrong if we do not teach and expect responsibility.