There are several islands of lines in the posted image where labels would easily fit.
Sure, but that's assuming the label engine works differently than it is documented for what you've commanded it to do.
When you tell the system to put labels on lines, it tries to fit the label where it will fit along a line, curving it if need be. It tries to fit labels subject to some constraints, as discussed in the Labels topic.:
When curved labels are automatically created and placed at the location of their originating curved lines, Manifold tries to place those labels so that the curves in the line do not force the label to conform to acute changes in direction, to avoid bending the text of the label so much that adjacent characters running into each other creates an unpleasant effect or impedes legibility.
The order of priority for placing line labels is:
- Center of the line - Place the label near the center of the visible portion of the line.
- Prefer straight segments - Place the label near a straight line segment that is long enough to fit it. If there are multiple such segments, use the segment closest to the center of the visible portion of the line. Text that runs in a straight line is more legible.
- Avoid acute bends in the label - If there are no straight segments reasonably available for placing the label, then place the label on multiple segments, but avoid placing the label onto a sequence of segments with high curvature, that is, with angles between adjacent segments greater than the Bend angle specified in the Position tab of the Style Dialog.
- Multiline text, box, or icon - Labels with multiple lines of text, with a box, or with an icon are always placed near straight line segments without curving the label.
Another such constraint is to avoid overlaps, but the above factors also come into play. If those contour lines have many acute bends without enough room to place labels, the label won't appear in such spots, even if overlaps with other labels are not an issue.
There are straightforward ways of dealing with the above. Use the Transform: reshape - smooth template to write a simplified version of the control line drawing into a new drawing, and then create labels from that, using those labels in the map. A few minutes of tinker time will likely help dial in the smoothing tolerance that produces more labels like you want that are in accurate enough positions to be useful.
Auto-positioning labels, or having options to do so that can be varied for different situations, is a never-ending source of enhancement opportunities for GIS packages. What ends up happening is an accumulation of very many special cases, along with the addition of extra capabilities, such as positioning a line label not on the line but somewhere else and connected to the line with a callout.
The usual drill is that GIS packages do something that works in most situations and then as users ask for improvements in various situations, those that are prioritized by the community get added. Manifold does that too, as discussed in the Suggestions page.