Goldfish will breed in spring, mine at work spawn any time between April and June. You will usually hear them splashing around at the water's surface and then see thousands of 1mm amber coloured eggs (infertile ones will turn white)
try feeding some high protein foods when the water warms up in spring...maybe some live daphnia, frozen bloodworm etc....this might just get them in breeding condition. Don't over feed though....just as much as they'll eat in a few minutes, twice a day and do a few water changes on the pond.