They like poor soil and dry conditions so have you over done the compost and or feed?

also they typically flower nearer June. especially if it is the 'single' stemmed type. even the smaller dianthus rockery type are less likely to be in bud. newly bought ones are brought on into flower by the growers early so we buy them sooner rather than later.

Sunny, free-draining soil. Don't feed them except a little tomato fertiliser (potassium), if that doesn't work, you could try taking cuttings and bin the old plants or throw the lot away and start again with new ones.