The lead artist in conjunction with the art director and producer must evaluate the performances of the art team. If the team is not meeting expectations as a whole, then the leads must make them aware of that and try to motivate the team to try harder. It is important for the art team to meet together once a week formally to check in with each other, evaluate their progress and provide feedback from their perspective on what they are doing to meet their goals. Nothing says you have to meet in a room on a sunny day. Taking the weekly meeting outside if it is nice, is always a good way to relax the team and provide a fresh setting from the confines of the production room.
Support takes many forms.
Supporting your team as their manager should be your priority. If an artist is having technical problems with their computer, it's your responsibility to make sure they report to you any problems they have that are affecting their productivity. In turn, you should work with the producer to get the tools necessary to get the job done. If an artist is having trouble working with a fellow team member, they should always feel comfortable enough to come and talk to you in confidence as their lead, and know that you will be able to help them deal with it. In turn, you may have the unpleasant job of weeding out the team members who are not consistently performing or are causing personality problems.