trying to understand the samtools flagstat file after alignment with minimap2

[sparthib]

I have a flagstat file here that looks like this:

flagstat
21384 + 0 in total (QC-passed reads + QC-failed reads)
14485 + 0 primary
6899 + 0 supplementary
16790 + 0 mapped (78.52% : N/A)
9891 + 0 primary mapped (68.29% : N/A)

I guess I don't understand why a primary read would be unmapped, my understanding is, the reason a read is primary is because it is mapped to a reference genome/transcriptome?

Any insights will be appreciated.

Thanks!
Sowmya