Field filters for nested custom queries


I have a nested question that I’m struggling to apply field filters to. The first question is a custom SQL Query returning data into a table, and the second question is built through the question designer to create a graph from the data returned by the first question.

My issue is; when I try to add a field filter on the dashboard the second question is on I can’t link the field filter to the fields on that second question. Or atleast, I can for some but not for others.

Anyone know why?