Free atom calculations can be kind of tricky. Here are a couple of pages that were helpful to me:
https://cms.mpi.univie.ac.at/vasp/vasp/Atoms.html
https://www.vasp.at/vasp-workshop/handsonI.pdf
I would recommend breaking the symmetry of your box. Instead of 15 Å x 15 Å x 15 Å, for example, try 14 Å x 15 Å x 16 Å, for example.