After wintering over, my mandevilla has loads of buds but most only partially open. The plant itself was trimmed way back and spent the winter in the house. It has grown into a beautiful plant after 2 feedings of Miracle Gro but flowering is a big disappointment. Last summer the plant and flowers were gorgeous.
Most likely to much Nitrogen, this will cause lush greens and little flowers.
You can add Phosphorous. Bone Meal is a good way to do this.