Naazuk,
Every heuristic runs according to low level code.
Some of heuristics terminates completely without giving any result if there are certain master data is not maintained.
Example: Planning of Standard lot of heuristic terminate completely if lot size not maintained
Where as same heuristic doesn't terminate if source of supply is missing , it just ignore that SKU.
So you have look after what algorithm written for the heuristic and how it works as well as what are the mandatory option to maintain not to terminate it...
Regards
Debashis