Flowers I want to grow but can't!

I'd like a garden filled with sunflowers, poppies, cornflowers - nothing unusual there. However, I fail miserably with them all. The sunflowers end up about a foot tall with a flower no larger than a dandelion. Poppies that apparently self seed everywhere for others, put in a meagre show then disappear. Cornflowers, those bright blue flowers that are included in Easy-grow packets for children, are not so easy for an old biddy like me.

Am I alone or do others struggle with certain flowers?