I had the same problems last year with my dianthus. They waxed and waned throughout the season. Last summer was my first gardening experience (3 10" pots on my balcony rails). I planted strawberry cream dianthus w/ begonias and vinca. Plucking the dried/dead buds off seemed to help. The flowers don't last long; I plucked the dead ones off, too (and the dried ovaries containing the seeds). Sometimes the plants looked practically dead, then suddenly came back with more color and flower. They took turns doing this (i.e., not all dying out at once). I'm not planting them this year.

As far as I know, dianthus are more seasonal plants - generally spring - early summer (and perhaps fall) and they prefer cool weather. I expect that perhaps dead-heading and some pruning might allow them to do some rebloom but I wouldn't count on them as a summer-long type plant. The carnation or sweet william side of that family might do better over the long term, although they still prefer it cooler too I expect.