1. Check to make sure each of the sub projects can be opened from PPM to MSP. If any of the sub projects have child projects of their own, then also check those sub projects to make sure they open successfully.
One known issue with sub projects that causes this issue with a master project is if either the Standard calendar or a team member's calendar is 51 characters or longer. If this is the cause, you will see the following error when attempting to open that sub project in MSP: "An Import error occurred. The element in the element with = <ID#> has invalid data. The calendar name does not exist. Click a calendar in the list, and then press Enter". Reference TEC1532415 for more information on this issue.
2. If all of the sub projects open successfully, see if removing all the sub projects from the master project, then adding them back to the master project resolves the issue.