We try to supply aged seed. Often this can be broken with GA-3. Called 'two-step' germinators by Deno. Some of these have unformed embroyos which must first develop at warm temperatures. Some seeds are '70 - 40' germinators, which need warm moist, then cold moist, and germinate at cold temperatures.
Many seeds are 'multi-cycle' germinators, needing several cycles of cold and warm periods.
Soaking the seed in shallow water and changing every day for several days will leach out inhibitors. Heavy daily waterings of pots may work. Some have oily coats and need detergent or peroxide soaks. Some seeds need the daily change from warm to cold that occurs in spring and fall, and they germinate best sown in pots and exposed to outdoor conditions. Get our Gibberellic Acid Information Sheet for brief tips on experimenting with these treatments.