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Any three lines drawn so that they connected at their ends will automatically form a triangular face, because their three shared vertices must always be 'coplanar'.
But if you draw four separate lines that join at their vertices, then their four vertices might not be coplanar, and then no face can form.
This can be resolved by drawings a 'diagonal' line to form two triangular faces from those four edges - because the two sets of three vertices must again be coplanar and thereby support a triangular face.
If you need 'rectangular' faces then use the rectangle tool to draw them - this guarantees that the the face forms. This also works for circles or polygon faces. OR you can draw edges snapping to the same larger face [perhaps within a group] so that the result is a loop that will 'face'.
If a edge needs by a face is erased the face vanishes, it can be remade by redrawing the edge.
If a face is selected and then deleted, then provided that all of its edges remain it can usually be recreated by drawing over one of those edges; some very complex outlines [especially those with inner-loops forming 'holes'] might not 'auto-face' on overdrawing an edge - in that case draw one or more diagonals to split the loop and get two or more faces added, these unwanted diagonal lines can be erased afterwards without compromising the integrity of the remade face.
If you do not draw a full 'loop' of edges then no face will form, since an 'outer_loop' is needed to support any face, no matter how simple or complex its outline is...
