If your composite decking boards have moved on the joists, there are a few steps you can take to fix the problem:

1. Identify the cause: First, try to identify the cause of the movement. It could be due to expansion and contraction caused by changes in temperature and moisture levels, or it could be due to improper installation.
2. Check the joists: Make sure the joists are properly spaced and level. If they are not, adjust them as needed.
3. Re-position the boards: Gently re-position the boards back into place, making sure they are properly seated on the joists.
4. Secure the boards: Secure the boards to the joists using composite deck screws or hidden fasteners, making sure they are spaced properly to allow for expansion and contraction.
5. Consider adding additional support: If the problem persists, consider adding additional support, such as blocking between the joists or adding extra joists.