Answer: I can only think of two possible causes. If these single blooms are the first blooms you have had on the plants since purchasing them, perhaps the plants are not the type you intended to purchase. Nursery tags can get lost or switched.
If they have bloomed double in the past but now are producing some single flowers, it is possible that a "sport" or genetic mutation has occurred and the shoots that results from this sport will continue to produce single flowers.
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