For tips on increasing participation, see:
How do I maximize student participation in Ascend & Elevate? Or
If you use those strategies and still find that there are specific groups who are underrepresented among your respondents, consider doing targeted outreach. For example, if implementing Ascend or Elevate, reach out to individual students who are members of that group and tell them that you’d really appreciate hearing their perspectives and remind them that their responses will remain confidential.