All the sheets in the navigator use the same font. Linked sheets are not bold.

One possibility is that one particular file is damaged, so Prism has lost track of the links. One thing to try in that case is to create an empty file, then use File Merge to bring in the damaged file. That can sometimes fix things.

If the problem is with all files, then the problem lies in the Windows control panel. Open the Display Properties dialog (right click on the desktop), go to the Appearance tab, click Advanced to open the Advanced apprarance dialog, and then choose Icon in the Item list. With some font selections here, the navigator loses its ability to show linked sheets as bold. In particular, the Arial font seems to not work well here. Tahoma is the default font, and it works well.