I like to think that a gardener is able to grow anything. I'm not saying I'm a good gardener but so far, most seeds that I have sown, even lychee and orange seeds, germinate into seedlings. The difficulty is getting them to grow and thrive in our tropical climate.
This year, I sowed two batches of coriander (cilantro) seeds and none of them germinated. Thinking that the first pack that I bought from the supermarket must have sat on the shelves for too long, I bought another batch from the seed wholesaler.
I tried all ways and means to grow them. I soaked some seeds in water and even crushed some lightly to break the tough layer of husk before sowing. Nothing worked.
It was puzzling because my friends scatter the seeds on the ground and they grow effortlessly. I thought maybe the conditions are not right at my house but a gardener doesn't give up so easily.
I begged my friend for some of her seeds (I didn't beg. She offered them happily.) and guess what? All the seeds that I've sown germinated in a week. It's too good to be true.

At this point, I believe the quality of seeds is very important. The ones that have been languishing on the shelves for too long will no longer germinate. For me, I prolong mine by keeping them in the refrigerator. They're still good even after one year.