The Copyright owner can reserve the right, in full, to file suit against a potential infringement on the work. This is part of the bundle of rights that can be conveyed in a license agreement. Failure to address this issue in the agreement can cause a potential dispute between the owner and licensee if a suit is filed.
A non-exclusive licensee would likely never be able to file suit for infringement. But the extent of a license is not always as clear as the title of the agreement may suggest.