This has been happening often when I use HitFilm. I'm not exactly sure how to describe it, so please bear with me.
In the viewer (next to the layer) there should be a few options available to me the angle of the view (top, left, active camera, etc) as well as controls on the right for zoom, dolly, rotate, etc. At first when I want to use those controls that are available but if I end up using any more than 1 view and then change back to a single view, all those options go away. If I then choose, for example 3 views, then they are available to me on view 2 and 3 but still not available on 1. I'm not sure if I'm making myself clear or not and if I am I'm not sure if this is a bug or perhaps something I am accidentally pressing or not pressing to get those controls to return. Any help on this would be greaty appreciated.