based on your description i can’t think of any obvious setup issues … here are a few general / brute force things to try, if you haven’t already …
at the player/pi end, try restarting the service (sudo systemctl restart plexamp, assuming it’s set up as a system service) or even rebooting the device
at the controller end, sometimes disconnecting from the cast device, refreshing the cast device list, and reconnecting can help (in addition to closing/restarting the app)
if none of this helps, then posting Plexamp logs (probably from the Pi ?) might help the Plex devs look into it further
