Unable to submit expense reports against a project code when employed by a different business unit

We are a large organisation with 5 or 6 different business units. Employees who are employed by business unit A can submit an employee expense against a project code, provided they are listed on the project, but this does not work for employees employed by business unit B, although they are listed on the project they are unable to select the project codes boxes to enter the data into on the create expense item page.

We are currently using version 24C