I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
Make sure your plants are getting plenty of sunlight as this is essential for good blooming on a Mandevilla. Try fertilizing them periodically with a soluble plant food such as Miracle Grow at the label rate. This should get them blooming again soon.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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