So sorry there is not a chorus of "This bud's for you" but it sounds like your Cosmos either had too much shade or too much high nitrogen fertilizer so that they made great foliage and stems at the expense of flower production. The clue is that once you fed with a plant food for flowers you found a bud. Seeds started indoors sometimes grow tall and leggy from lack of sunlight so this slows down the bloom cycle. This may be why the self seeded Cosmos bloomed when your indoor-started plants were late bloomers. Don't give up. Gardening is a learning adventure and you may want to save the seeds from your blooming Cosmos and try seeding these directly into the soil in late spring rather than starting the seeds indoors. I see a rainbow of blooming Cosmos in your future. Keep growing, Marianne Binetti