Well coreopsis do not naturally bloom all summer. They will bloom for weeks at a time, but they are not like annuals that will keep their color all summer long. Also, they won't bloom well if they are not getting enough sunlight (at least 6 hours is required) or if they are being overfed with a nitrogen fertilizer. Also, it often helps, after the first flush of bloom, to cut coreopsis back a bit after the flowers fade. This will stimulate new growth and another flowering period.