A not working box projector could have several reasons:

• The target mesh has no collider component attached or the decal has the Collider Only (constraints tab) option enabled.
• The collider and the MeshFilter component are not on the same level and Recursive Lookup (constraints tab) is disabled.
• The mesh is set to static and there's no MeshCollider component attached. Relevant in play mode or for decals that are not baked.

More information about projection targets: http://nu.sycoforge.com/learn/easy-decal/projection-targets

More information about performance: http://nu.sycoforge.com/learn/easy-decal/performance-considerations