We have small flowered native Tropaeolum species in Chile (Tropaeolum tricolorum, T. azureum, T. ciliatum. T. hookerianum and many more). They are all perennials (form either a tuber or have fleshy roots). The size and appearance of the seeds is almost identical to T. peregrinum. I sow mine outdoors during Autumn, in a soil mix with plenty of grit or sharp sand to provide good drainage, which is essential. Rain has become very scarce for the last 6 years, so the soil is barely moist. I do not nick the seeds, nor do I let them swell in water. Maybe a tiny bit too much water is your problem.
I grew T. peregrinum some 10 years ago and had very good germination rates, Good luck with yours!
