P6 R16.2 Version not able to add pred or succ via detail box

I experienced this when switching my laptop from having one monitor to two monitors back to one.....  It seems that when I moved P6 to the new monitor, sometimes the ancilary windows move or I moved them to the new monitor.  When I then went back to just one monitor (just my laptop monitor),  the detail windows apparently appeared on the added monitor and were just sitting there, waiting for me to respond.  I now have a backup spare monitor that I'll plug in to find the detail windows and move them back.  I also have tried lately to move all windows back to my laptop monitor before I shut down....  See if this helps!

Try installing the latest patch. Usually it helps. 
There are several known bugs including invisible relationships buttons that you just described in version 16.2. For me resizing the buttom layout would do the trick.