Table Joins - using Crystal Reports XI:
  1. Verify that the table joins in the subreport are set to Left Outer Join. (BB339584)
Subreport Linking - using Crystal Reports 8.5 or XI:
  1. Right-click on the subreport and select "Change Subreport Links".
  2. Verify that the field linking the main report to the subreport is correct.
Other things to check:
  1. Verify that the correct conditions are entered in the Select Expert (Report > Select Expert).
  2. Re-export the data from Raiser's Edge and refresh the Crystal report.
  3. Verify that the subreport is referencing the correct export file (.mdb file). (BB406610)
  4. Check the conditional suppression formula on the section missing data (Right click section > Section Expert > X2)
For detailed steps and additional troubleshooting tips, please review the following Knowledgebase Solution:

Records, classes, or data are missing - when running a Custom Crystal Report (BB23774)