Incomplete casting can be due to:: (A) Insufficient venting., (B) Inadequate or too small spreu., (C) Low casting pressure., (D) All of the above.
Answer is D. Reasons for incomplete castings:

Insufficient venting
Inadequate sprue (too small sprue)
Insufficient alloy used
Alloy not sufficiently molten or fluid
Mold is not heated to casting temperature
Premature solidification of alloy
Low casting pressure