Resource units not getting populated for WCs which have not the exact same shifts as Mfg Constants.

 Hello,

We are trying to populate the resource units for the Work Order back scheduling but noticed that when we run R3007G, resource units are only getting populated for the work centers which have exact same number of shifts. We have 3 shifts of 8 hours each in Manufacturing Constants, so the resource units for WCs with only one shift of 8 hours doesn't get get populated on running the refresh resource units program. When we changed the shifts to 3 each of 8 hours, we can see that resource units are getting populated. 

Is there some set up we are missing and that's why this anomaly?