OTDR Sections with no slope measurement

Why do some of the OTDR section results have no slope values and others do on my Viavi OTDR?

When the section is very short (2Kms or less typically) and very linear then the intrinsic error starts to be come high when compared to the very small db/km loss result making it difficult to ensure an accurate result. Is this scenario the test will not report a db/km value. Other factors can also contribute such as insufficient data points for that section. In these cases, the customer can perform a manual slope measurement for that section.