It could be that in your climate you aren't getting enough natural stratification from winter temps? Cold temps for a period of time break the seed's dormancy. Agastache does best with moist cold stratification. You can refrigerate the seed in damp spagnum for 30-90 days. If the seeds sprout in the fridge, plant right away. If you have cold enough wet winters, the seed will naturally break dormancy. If you don't want to bother, buy plants as they tend to multiply nicely on their own.
Love your signature!
Buy locally, buy only humanely raised animals, eat in season, keep bees!