lookupRdrNameInModuleForPlugins with constructors

I'm trying to construct a dictionary in a GHC plugin. I'm stuck on finding the constructor for the dictionary. When I use `-ddump-simpl` on the module that defines the class, I see "Circat.Rep.C:HasRep". To try finding that constructor, I say

lookupRdrNameInModuleForPlugins hsc_env (mkModuleName "Circat.Rep") (mkVarUnqual "C:HasRep")

However, I keep getting `Nothing` as a result. (Same without the "C:".) I've also had this same difficulty when looking up constructors for algebraic data types and when looking up TyCons. For regular value Ids, lookup succeeds. What am I missing? Thanks, - Conal