Using the Red marker-squares helps detect a "flat" surface or plane so that a model can be placed in a stable position. Sometimes however 'floating' can occur between moving between Dolls-house and Life-size views or on outside surfaces. Also reflective surfaces (e.g. glass tops) and brightly-lit white surfaces can be problematic.
In all cases, if floating occurs, tap the red-Realar icon in the bottom-middle of the App, reselect model and return to plane finding using the Red-marker until a solid square appears.
Protip: try and make the Red marker-square appear three times: the more time spent detecting plans the more stable the model.