Until the gate freezes, holding pressure pushes more material into the cavity to compensate for shrinkage. A holding time that is too short throws that chance away.
To find the optimum holding time, weigh parts made with increasing holding times; the point where the weight stops increasing is the gate freeze time.
If settings don't solve it, the part design (wall thickness, ribs) or the gate location needs review.
This note is general guidance. For exact settings, the material datasheet and the machine maker's instructions are the reference.