  During a decade of the $Fermi$-Large Area Telescope (LAT) operation,
thousands of blazars have been detected in the $\gamma$-ray band. However,
there are still numbers of blazars that have not been detected in the
$\gamma$-ray band. In this work, we focus on investigating why some
flat-spectrum radio quasars (FSRQs) are undetected by $Fermi$-LAT. By
cross-matching the Candidate Gamma-ray Blazars Survey catalog with the Fourth
Catalog of Active Galactic Nuclei Detected by the $Fermi$-LAT, we select 11
$\gamma$-ray undetected ($\gamma$-ray quiet) FSRQs as our sample whose
quasi-simultaneous multi-wavelength data are collected. In the framework of the
conventional one-zone leptonic model, we investigate their underlying physical
properties and study the possibility that they are undetected with $\gamma$-ray
by modeling their quasi-simultaneous spectral energy distributions. In contrast
to a smaller bulk Lorentz factor suggested by previous works, our results
suggest that the dissipation region located relatively far away from the
central super-massive black hole is more likely to be the cause of some
$\gamma$-ray quiet FSRQs being undetected by $Fermi$-LAT.
