Adding materials to different regions

Hi, 

 

I want to add materials to different regions of the FEKO union geometry. When, I right click the region of interest and go to properties  only 2-3 medium available from the region medium drop down menu. Even, I add a medium by using the + sign, the medium only goes to media tree but can not select from the drop down menu.

I did  not have this issue with simple non-union geometries.

Please share if you know how to solve this problem.

 

Thanks in advanced.