Mirror image disappears when joining with another object Before joining.

After joining.

The Cross and surroundings are half mirror image. But when I join it to the back wall, the mirror image part is disappearing. 
Please tell me why and how can I solve this problem.
Thanks.
Objects are not carrying their modifiers. After joining them, only the main object's modifiers will be valid. If you have a mirror modifier on the cross apply it first, then join them.