Hi,
This could be an issue of data rights i.e. if a data is created by users at root business unit and user from child business unit tries to access data of a parent business unit, then offcourse, he won't get any results. but he should get his own created records.
Secondly, Go and check that your user should have read rights on his parent business unit as well. in this case he will be able to access all the data.
3rd reason could be that, when you add a custom report, crm automatically adds a filter of returning just the records which are updated within last 30 days. you need to specifically remove this filter.
anyways option 2 may fix your problem... ... ...