How can I get PostgreSQL to follow up filtering jobs that my FDW ignored?

I'm making a FDW (Foreign Data Wrapper) in PostgreSQL for myself using C. It seems to be working rudimentary, but I have one problem. When I pass it a query like the following via psql:

select * from FOREIGN_TABLE_A where COL1=1 and COL2=1;

...then the FDW ignores the second search predicate (COL2=1). How can I make PostgreSQL use my second search predicate? I tried debugging but PostgreSQL never returned any useful results. Are there any rules for enabling PostgreSQL to use multiple predicates in a FDW in GetForeignRelSize(), GetForeignPaths() or GetForeignPlan()?