Are they Enterprise USers? AD? SAP?
kindly try the below things:
Run UMT with following migration options -
- Incremental
- Live to Live (Eg. 3.1 SP3 P5 to 4.0 SP5)
- Select All Users and Groups
- Select all custom Access Levels
- Assume all dependencies are automatically selected. (IE inbox, categories, favorites)
- Select folders which you would like to migrate
- Override Objects Rights