Here's the scenario:
In an elevation or section view, you are dragging the model-end of a Level, and you notice it will not drag past a specific point. Some other levels work, but this particular level[s] just quietly snaps back to the previous point.
This occurs related to the shape and orientation of the specific Level. Because a Level in Revit is actually a rectangular plane, if a Level is added in a view [such as a section or elevation] not parallel to the N/S or E/W axis, Revit is essentially looking at a rotated rectangle in other views.
Adjusting a level extent moves one corner of the level's rectangle parallel to the view in which the adjustment takes place. This can lead to a degenerate rectangle, and that's what places limits on the extent of adjustment.
In the non-parallel view [where the level was originally added] you can adjust the level model end to any desired point since you are viewing at the same plane.
However in other views [such as the default N\S & E\W Views], you would be viewing the level ends projected into the current view plane. The following image better illustrates this concept:
At any time you can toggle the 3D Extents > 2D Extents to graphically override the Level appearance in the view. This would allow you to drag the end to any desired point.
Special thanks to Steve Bianco for the background and example image on this behavior!