Trouble diagnosing why my flow isn't working - how can I trace an auto-launched flow?

Hello,

I am trying to build an auto-launched flow that will update a specific subset of records - and when I test it, nothing is happening and I am not recieving an error or fault of any kind. Clearly there is a breakdown in my logic somewhere - but I can't figure out where without getting a fault notification.

I can't add a screen to see if my Fast Lookup is finding any results, because the flow is triggered via process builder. Is there a way that I can view the detail that is output when a fault occurs, as this would be hugely helpful.

A little about my issue:

I have a Master-detail relationship between two objects called "Campaign_Plan" (the parent), and "Media_Buy" (the child). Because of our business logic there is often a case where the child is created before the parent (I have this master-detail relatioship set as re-parentable).

I have a separate process in place where if the child is created before the parent - the child gets attached to a Temporary Campaign_Plan record. That part is working fine.

What I want to do is set up a process where whenever a new Campaign_Plan record is created, all Media_Buy records that may belong to it will be re-parented to the new record.

So I have a process-builder process in place that states:

Whenever a new Campaign_Pan is created, trigger the flow "Re_Parent_Quarterly_Buys" and set the ID of the newly created record as flow variable NewCMP_PlanID.

From there, the flow looks up the Campaign_Plan with the ID that is NewCMP_PlanID, pulls the variables "Campaign" (lookup, ID) and Broadcast_Week and goes into a fast lookup. I have pictures below to outline the rest of the process. Can anyone help? Thanks in advance!