Look at the cushion first: if it is zero, not enough material is being dosed and the shot size must be corrected.
If there is a cushion but the part still doesn't fill, check speed and the melt and mold temperatures. Long flow paths and thin walls need higher injection speeds.
Burn marks at the end of fill mean trapped air: clean the mold vents or make them deeper.
If the cushion slowly drops over consecutive shots, the check ring or barrel is leaking back.
This note is general guidance. For exact settings, the material datasheet and the machine maker's instructions are the reference.