How to detect: In the 3D View and photo projections the Bent Tube does not appear at all, but other Bent Tubes do appear.
How to solve: a) There may not be at least two marks on oriented photos, b) the tube is misreferenced, c) the angle between photos, where the tube is marked, may be too small, or d) the marks’ photos do not rotate around the axis of the tube. Replace one of the marks (or add to the existing marks) a Bent Tube Mark on an oriented photo with bigger angles to the other photos and that rotated around the tube. Check the tube’s references.