I am using process builder to update child records from a parent object. But its not updating records of one specific customer child object. However when in same  process builder I am trying to update another objects child records  then its getting update. Any idea, is there any limitation to update child records in some scenario. 
In action type I am selecting update records and then in Select a Record to Update I can see child object suffixed with __r, then I am selecting the column and assigning the required value but its not working.
Same scenario I applied for another object and its working fine.

I am using process builder to update child records from a parent object. But its not updating records of one specific customer child object. However when in same  process builder I am trying to update another objects child records  then its getting update. Any idea, is there any limitation to update child records in some scenario. 
In action type I am selecting update records and then in Select a Record to Update I can see child object suffixed with __r, then I am selecting the column and assigning the required value but its not working.
Same scenario I applied for another object and its working fine.