Verify the staus of the corresponding standby database.
if it is not acknowleding redo from primary, there is a chance that your primary database will stall or go down to ensure there is no data loss.
Quote "To ensure that data loss cannot occur, the primary database will shut down, rather than continue processing transactions, if it cannot write its redo stream to at least one synchronized standby database"
Section - 1.4 Data Guard Protection Modes - Data Guard Concepts and Administration 11g Release 2 (11.2) E10700-02
From section 5.1 of the same document ( Maximum Protection )
"Oracle recommends that a minimum of two standby databases be used to
protect a primary database that runs in maximum protection mode to prevent a single
standby database failure from causing the primary database to shut down."