It is possible that G+ posts may be failing because that profile activity in the application has reached the limit that G+ has set in place. For example, viewing the Inbox where G+ is connected or setting up a post and accessing your circles are both actions which can cause you to reach your limit. If you do receive a notice that a message is failing for G+, please try posting it directly on your page or try posting after waiting up to an hour. The post limits are established and enforced by G+, so Sprout Social has to stay within the set limits.
Sprout Essentials Guide