sheet index - numbering using text fields

Hello,

One of my .dgn file have sheet models in it. There are 17 sheet models, all of them are added to "sheet index".

In every sheet model I have title block with text field which automatically should add my sheet number, taking it from sheet index properties.

The problem is, it worked only once I set it all up. When I was trying to create another .dgn file based on my first .dgn (just copy/paste, and edit that new file), my text field has stoped working properly. In the place where should be total number of sheets models there is only hash sign "####" (I have text field in "no. of Total number" format eg 1 of 17). When I try to edit that field, pop-up information appear "Unable to display properties of selected target.".

Is that connected to Workset settings, maybe? Or I'm doing something wrong. My goal is to set automatic sheet numbering in my title block using "text fields".