Hello!  I've been working with Airtable for a year and have built lots of automations and formulas. I'm stuck though and hope someone can point me in the right direction! 

I've got a base with Table A and Table B. 
Table A is a list of todos which are linked to clients who are listed in Table B, so each client has multiple todos linked.
Table B also has drop-down list of all of the stages that the client goes through.
Table B's primary field is their unique client number. 
Table A therefore has a linked field with that identical number in it. 
Table A also has another field with the same unique client number.

I am trying to write an automation which will do this:
- When a record in Table A is marked "done" (or other trigger depending on the type of todo)
- The drop down for that client in Table B is updated. 

This seems like it should be simple...
- Create the trigger in Table A's Todo record
- Use Find Record to find the record in Table B where the primary key = the client number from Table A
- Update the record found in the Find Record step of the automation.

But it won't let me create such a thing!  I have tried numerous ways to set triggers and Find Records, and no matter what I try, the client number fields in Table A are greyed out as "not valid" to be the dynamic lookup for the "Find Record" step. I cannot figure out why!

If I choose a different field in Table B to look in (not the primary key field), it'll let me select the client number field from Table A to lookup. That doesn't work of course because the client number isn't in any of them... but the client number field of Table A is indeed an option. But as soon as I tell it to look in the primary key field of Table B, all of a sudden the client number field of Table A isn't a valid option. 

Why is it doing this? What do I need to understand about how Airtable automations work? Why is it not simple to use one record to update a separate field in a linked record?