STC - Spot Elevation - Answer

As Afaq says in his comment, a color fill can interfere with placing a Spot Dimension. The other thing that can interfere is Model Graphics Style: Wireframe. A Spot Dimension can't "see" a surface if you are using Wireframe. It won't mind having a color fill or using wireframe after you place the Spot Dimension but you won't be able to place one while either condition exists.