The discussions between labor and management ended inconclusively.

Literal

Labor-management [between-間の] talks [topic-は] vaguely ended.

うやむやに終わった ('ended vaguely, fizzled out') is a set phrase — うやむや means 'vague, unclear, inconclusive.' 労使間 uses 間 ('between') after the compound 労使 ('labor and management').