Some Task never completely finish in Threading.Tasks.Task.Factory.StartNew

Hello All, I am implement multithread in one background service ,

but i found some thread task assigned method never finish randomly ,Could you please help me here 

var taskArray = selectedInventory.Select(Inventory =>
    System.Threading.Tasks.Task.Factory.StartNew(() => {
        ProcessData processData = new ProcessData();
        processData.SomeCalculationmethod(Inventory);
    })).ToList();

System.Threading.Tasks.Task.WaitAll(taskArray.ToArray());

I'm passing Inventory data with 5 record ,once finish all 5 task then it check and pick 5 record for process .

In Some cases randomly few record task is not finishing inner method of SomeCalculationmethod compeltely 

Althought i have also used WaitAll to wait all task for complete