Check the runtime logs. Google for where they are located on your machine.
You’d be looking for things like exceptions or errors in the log.
OR if this is just a UI Anchoring issue, here’s some notes:
Here are some notes on UI Anchoring, Scaling, CanvasScaler, etc:
Usually you need to choose a suitable ScaleMode and MatchMode in the Canvas Scaler and stick with it 100%. Generally if you change those settings you will often need to redo your UI entirely.
I also use this CanvasScalerOrientationDriver utility to make sharing UI for Landscape / Portrait easier. Read what it does carefully.