Why can’t I get split screen to work on my ipad pro 9.7”?

I have tried numerous times to get 2 apps to sho up in split screen. But it doesn’t work. I have an ipad Pro 9.7” and it won’t put 2 apps in equal halves of the screen. One app is always at least ⅔ of the screen while the other app is only ⅓ of the screen. I’ve tried to pull the edge of the ⅓ screen to move to ½ of the screen but nothing happens. There isn’t any place on the ⅓ screen to resize the screen on that app.