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