Automatic Email workflow when a formula field is updated

Master Table contains child table records.
Master Table has one formula field and value as initially “Incomplete” and once the child table records completed, the formula filed value will change as “Completed”.
I set a workflow on Master Table with “ALL_CHANGES” and tried with different type of conditions like

[Inspection Status] = “Completed”

[_THISROW_BEFORE].[Inspection Status]<> [_THISROW_AFTER].[Inspection Status],
[_THISROW].[Inspection Status] = “Completed”

There is no system error but workflow not triggered when the value has been changed. In Audit log, there is no such event occurred.
The below is the formula to set value for “Inspection Status”.

IF(COUNT(SELECT(Inspection Template[Checkpoint], [Fleet Type]= [_THISROW].[Fleet Type])) = [Questions Attended], “Completed”, “Incomplete”)

Please advise.

Instead of triggering the workflow rule based on a update to the Master Table, why not trigger it on changes to the Inspection Template table? If I understand correctly, it is the completion of the last of those inspections that seems to be the real triggering event.

Indeed, you are right. The child table has no. of questions for the particular inspection record from the Master Table. The no. of questions may vary depends on the equipment type. If i make a workflow on the child table, is it correct way to do that? If so, can you tell me the logic to be applied?