split screen not working correctly

ipad air 12.3.1 split screen does not work, but slide over does. I can put any app over any app but it will not "stay" in split screen mode. Yes, pull down or up on the top bar, but only springs back to slide over status. While in slide over mode, the background "main" app is greyed-out until top app is slid off the screen. Any suggestions? reset, reboot, etc already done..