3D FlipBook - Added Elements not showing (PRO)

Hi,

we have generated a flipbook from a PDF. Works fine. Now adding a simple button to jump pages as described here: https://www.youtube.com/watch?v=t_6pISxwtBs&app=desktop

Using your exact code. The Button is clickable but does not show. It is as if it is transparent. Any ideas on what might cause this? Here is a screen shot from firebug in Chrome: https://pasteimg.com/image/screen.43n9

Thanks