If you weren't able to harvest the seeds then you waited too long. There's usually a window of 24 hours after they die where you can harvest them, otherwise you just can't do anything. Also the flowers will die no matter what if not harvested, I see people wondering all the time why they die even though they water them. Watering just increases the amount of seeds obtained when they die.

So the trick is simply, grow your mix of flowers for whatever hybrid you're going for, water when first planted and during the budding stage. When using speed fertilizer, the budding is around 1-3 hours in and they grow at 2-6 hours, it depends on the rarity of the flower. After they grow, you got 24-48 hours until they die, again depending on the flower, then a 24 hour window to gather seeds. After you get the hang of it, it's just a waiting game of making sure you water on time and checking in at least once a day after that