I have a situation where an update to an object (objA) finds a Child Obj (objB), then updates a grandchild object (objC) related to the child. 

So far the Flow is completing all the steps, according to the debug log, but the grandchild object is failing to update. 

The process start from an invocable process in the process builder that launches the flow providing three variables, including the objA Id. The Flow is then launched with a DecisionStep:
DECISION - checkObjAId - checks if the varObjAId variable is not null. The Yes decision leads to a record lookup step for the child objB record

RECORD LOOKUP - FindObjB - Lookup objB where objA_lookup = varObjAId, 
                                   Action: assign objBId to varObjBId variable
Next I have another decision step to eusure the varObjBId value has a value

DECISION - checkObjBId - checks if the varObjBId is not null. The Yes decision leads to a Record Update step for the grandchildObjC record

RECORD UPDATE - updateObjC - Lookup objC where objB_Lookup = varObjBId
                                  Action: update objC field values with the other two variables provided to the flow from the ProcessBuilder. 
This step does fire, according to the debug log, but refreshing the objC record in Salesforce does not reflect the updated values.

Am I missing something obvious? I can add screen shots of each step i nthe Flow if it helps, but all the steps are firing according to the log.