To work with 'Copy item' action, 

• The source and destination lists/libraries should have the same structure else the workflow will you a error message that the source and destination are incompatible. And also 'Content Types' should be same (if any).

And also in order to copy multiple items from 'List C' to 'List B' in your workflow('List A') use 'Collection Operation' and 'For each' actions (i prefer to get items ids from List C) to pull information from 'List C' with some conditions and use 'Copy item' action to copy multiple items from 'List C' to 'List B'.

Also i prefer don't use conditional start in your workflow settings it may lead to cancel/fail the workflow if so many items found to copy from 'List C' to 'List B'. Instead set workflow option to start on item creation and inside workflow use 'Run If' or 'Set a condition' action to check any field value is equal to specific value and clear the value at the end of workflow.

‌ has given you a pretty good overview there.

• On item creation in List, query List C for items applicable to the task selected in List A.
• Store in a collection using the Item ID
• Use a For Each loop to iterate through the collection and copy the Item from List C to List B based on the ID of the current item in the collection being operated on.

Thanks for the advice.  Will include a 'Run If' or condition within the workflow rather than conditional start.  I think my problem was the two lists had different columns as I hadn't paid enough attention while building them.  Thanks again!

Hi

I have a very similar requirement.  I need to copy child  Items between two lists with each having the same columns.

However, I don't see a Copy Item Action in my Nintex for Office 365 Designer ( only copy document).   My initial thoughts are again similar to the approach in the answer above:

• Query child list items that are related to the current List item
• Store each child list row in a single output so a collection 
• Use a For Each loop Iterate through the collection and for each 'row', build up an Insert Item Action call  to  effectively copy the item from source child list to the destination child list.  

. I want to minimise having to store each column per each selected row in shadow variables  as in Separate output per column.  Anybody tried this?

Quick update: I have able to populate a dictionary ' row ' with each child list item but as far as I can see this isn't a way to readily use this to insert each  item in the destination list - makes a handy debug Log entry tho.  Will need to use separate column variables instead and then use these in each insert.