Management wants to make very clear that they are not going to be controlled or dictated to by the unions, while workers are very clear that they are the means of production. In a democracy there are many policies and guidelines that govern the workplace in terms of the environment and salaries and those give a lot of room for power-play between management and unions. Such negotiations tend, therefore, to be based on bargaining opposing demands rather than on meeting different needs and interests. The challenge is to help parties perceive the big picture. Often they each see it from their perspective only.
Management views the situation from their budget, from a profits perspective.
Unions, on the other hand, perceive that there is a never-ending pit of money, that the company makes sufficient money and is simply withholding it when refusing to compensate workers adequately. Workers also argue from a rights perspective. You've mentioned that you used caucusing quite effectively in this intervention. A mediator calls for a caucus when he wants to speak to parties separately. Caucusing is used when parties are not able to move beyond a particular point in the discussion for instance, if they are focussing only on their own perspective, are too emotional, aren't listening, or are worried about making too many concessions and 'losing face'.