Coreopsis is a true wild flower. That means they don't like to be tamed. They need plenty of sun to thrive. Look at how they grow in the meadows - hot, sunny, windy & arid conditions with very occasional rain. Try to provide similar conditions. I have "Early Sunrise." It gets about 18 inches tall, deadhead them for continuous blooms. When the plant starts to run out of steam, let it go to seed so you have plants for next year. The seeds are popular with birds too.