Datum planes, such as those for levels, grids, and reference planes are not visible in all views.
If the datum does not intersect a view’s plane, then it will not be visible in that view. To ensure visibility, the 3D extents of the level must overlap the crop region.
Datum planes are modifiable. You can resize their extents so that they are visible in some views, but not others. You can also change datum extents in one view and then propagate that change to any desired parallel views in which the datum is visible.