In the element flextables, the values in the flextable are not listed unless the cell is highlight, as shown in following screenshot.
It is possible that the font color is the same color, or similar to, the background color.
Go to Tools > More > Options, and go to the General tab. Change the read-only foreground color to something that contrasts the background color. Select Okay. The model data hopefully now appear in the flextables.