Why are the flower petals on my Mandevilla curling downward
Leaves are spotted yellow and green and just drop off. No curling of leaves. Do not detect any insect problem
There are several possible explanations for mandevilla leaves turning yellow. One of the most common reasons is improper watering. The soil must be kept damp but not soggy. Too dry and too wet soil will damage roots which then damages leaves. Another possibility is disease. There are several that attack mandevilla. Here is a good article describing these: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-disease-problems.htm. It does not look like a pest problem from the pictures but carefully examine the leaves, especially the undersides, for tiny insects. You may not see them move so it has to be a very deliberate examination. It is always a "best practice" to remove leaves that have fallen. If they harbor disease, the disease passes into the soil or puts other plants at risk when the wind blows diseased leaves around.