2
Possible one of two things or both:
Permissions - are the correct group permissions set to view the blocks.
Theme - The addition of bottom-center-left and right is a relatively new addition to XOOPS CMS, and not all themes have the necessary code in, to handle these blocks, especially on older themes.
If it is the later, and your familiar with basic HTML you can make the changes yourself. download a copy of the Mor.Pho.GEN.e.sis theme, and have a look at the code used there for center blocks, this should give you an idea of what to change/add.
Some of the older themes have been updated with the new block code, but I see yours is one of the older 7dana themes, I don't think any of them have been updated.
Hope this is of some help.