Why are microdeletions and microinsertions difficult to diagnose?

Microdeletions are very small a chromosomal deletion, usually smaller than 5 million base pairs (5 Mb). Because of that, it is hard to detect it by conventional cytogenetic methods ( through a microscope). Thus, the technique used for detection is high-resolution chromosome banding done by fluorescence in situ hybridization (FISH). The same thing is with microinsertions.