The core idea
Some bonds trade infrequently because an issuer has many separate securities, the issue size is small, investors hold the bonds to maturity or dealer inventories are limited.
What is happening underneath
Complex structures and weaker transparency can reduce activity further.
How investors should read it
A quoted price may therefore not represent the price available for a large immediate transaction.
The same market move can carry different information depending on which maturity, issuer and risk premium is changing. Read the question together with the underlying concepts rather than treating one price move as a universal signal.