What could be the reason if annealed primer fragments cannot be ligated into a vector?

If the primers in the ligation reaction lack a 5' phosphate group, the ligation efficiency is relatively low, but it may not completely fail.
If the phosphorylated product still cannot be ligated into the vector, it is necessary to check the vector digestion efficiency and review the primer design. If no issues are found, consider optimizing the primer annealing conditions and the ligation reaction system.