Double check your static mesh to make sure it's Accepts Static Decals setting is checked on.
Select you mesh in the level hit F4 to bring up it's properties.
Open the StaticMeshActor tab, then open the Rendering tab under that, Accepts Static Decals should be right there.
The DecalActor itself also has a Project On Static Mesh setting which probably needs to be on as well.
Bring up the DecalActor's properties the setting is under DecalActorBase -> DecalFilter.
Hopefully that helps, admittedly I haven't had much experience with decals so I might not know what I'm talking about here...