New to Plex - no movies will play (music will)

I have loaded up Plex on WD PR2100 NAS. I can see all the titles from all my registered devices (4K TV, Tablets, Smartphones)… Installed latest App update on WD PR2100 before i started… I get the spinning wheel of death that never goes away when trying to play any video content (HD or SD). I can play the content outside of Plex so I know the content is fine… but I am obviously missing some setting here. When I look on the server it thinks the playbacks are fine… no errors. Please help - I’ve wasted much of Saturday fiddling with this…