The problem - Team members were unhappy with what they perceived was an unfair bonus structure and it was causing disharmony in the team. They brought up the issue with the team leads who brought it up with their boss who took it up with his boss. However the buck stopped there and the problem continues to fester with no solution. The members are slowly getting demotivated.
As a leader, pick up signs of disgruntled employees and leave a door open for them to express their unhappiness. Once we realise that what they are saying is valid and is in the interests of the team, we must assure them of some action. This must come with some kind of a time line. If we do not get back to them with an answer or an assurance we will lose our respect as a leader. So it must be heard and acted upon and a final result given - maybe a partial address of their issue, or a real problem that management has and some alternate way of compensation or even just an explanation that they have been heard and under such circumstances, such and such measures would be taken.
For this the leader must fully listen to the team members and their grouse, see if it is valid, take it down to his boss, explain the seriousness of the situation, explain his angle fully without holding anything back, convince his boss or get convinced and get back to the team with an answer. If the team is likely to get no benefit, they must be given an assurance at least.
The fact that they have been 'heard' and that the leader has 'fought' for them will keep them engaged, feel valued and seen and heard. Don't leave them hanging.