Incomplete casting can be due to:
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
Insufficient venting.
Inadequate or too small spreu.
Low casting pressure.
All of the above.