Requested "resource units work" filled to some WC and others not

For a Work Order the warning message "resource units not set up" was displayed and WO routing dates were not updated when a change was made in the WO upper level. 

I noticed that the fields "resource units work" (image below) was missing in the P3007 screen for the WCs related to that WO, once it was recorded, the message wasn't displayed anymore and the routing dates were automatically updated. 

However, for other WO of the same branch (different item/WCs), this message was not displayed and the routing dates were successfully updated, even though these fields were blank for the WCs. I'm trying to trace which is the setup that requires it to be filled to some WC/Item and not to others.