The incomplete fill or Non Fill is one of common casting problem and you can check the following:

1. Casting temperature too cold.
2. Flask temperature too cold.
3. Insufficient Vacuum for vacuum casting. Check seals, gaskets, filter, vacuum lines and pump.
4. Wrong speed on centrifugal casting machine.
5. Inadequate sprues on the castings.
6. Incomplete burn out can effect air permeability of the investment.