The dimension symbol is probably used elsewhere, like the model or in a note or table or other dimension. If it is "otherwise occupied" it is probably not available where you want it.
To me, the logic behind this is quite pesky. It can cause things to pop up anywhere in a drawing if you remove it's "primary" callout. I don't know of any settings that would allow you to use a particular dimension symbol more than once in session. The fact that it allows you to overwrite only temporarily is even more troubling.
