Guys, I am pulling your legs including the NTUC part.
There is no such thing as a single importer. All retailers will occasionally give you the excuse that they only deal with one importer so that you go away if they do not want your product.
All retailers except for mum and pop stores have someone handling wholesalers and suppliers. In small stores, its the boss, bigger stores have buyers who specialise in various categories - food, cutlery, lines, electronics. Major stores have procurement department who controls all buyers within the company.
Returning products to the supplier or selling on consignment is part and parcel of a suppliers work.
When supplying to major stores, persistence is the answer. Be patient, be nice, have brochures, do regular visits to say high even if they do not buy anything.
There is no such thing as single importer as it does not make sense. You always have a number to protect your business. The only exception is when it is franchisee who has not option but to buy from the franchisor.