“Toy” as a reluctant adjective

Sometimes we hear a non-native English speaker say something like “That problem seems very toy,” where an American would probably say “That seems like a toy problem.” In fact, both speakers are using the noun “toy” as an adjective, but there are many restrictions on how an American would typically use it - only immediately preceding its referent, and without modifiers like “very.” Other English constructions exhibit this as well - for instance, we would say “That looks like a bottle cap” but not “That cap looks very bottle.” Is there already a name for this sort of restricted noun-to-adjective conversion? If not, I suggest “reluctant adjective.” :-)