This TED talk gives very good insight into the work we, at Open Minds, do on a daily basis. We work with our clients to uncover how they can "reduce friction and increase motivation" within their human capital.
How to reduce friction and increase motivation differs from personality type to personality type, so listen to the talk and think about where the friction is for your team and then what motivators you have in place. Think about the people who are not doing so well at the moment and perhaps ask them where their friction is and what would motivate them. Remembering that things like culture, processes (too much/too little), leadership style etc, can all create or reduce friction.
We recently worked with a large national bank where almost half their sales force were underperforming. After doing our research we found that, from a personality perspective, this underperforming group did not align with the banks culture and resulting leadership style and this was dramatically increasing friction and reducing motivation. By making the Bank aware of this and working with them to change behaviours, we very quickly improved performance.