I have 2 diplandenia plants, one is making runners really well but the other is not, why? They are planted in the ground in different areas of my flower bed that does get different amounts of sunshine throughout the day. Any idea why the one plant isn't making runners?
A photograph might be useful, but it's possible the plant you have which is climbing or 'making runners' is actually Mandevilla, and the other one is a true Dipladenia. These plants are often confused, even at point of sale, and do look fairly similar especially when young/small, but have different growth habits. Mandevilla is a vine and will climb up a trellis on its own, whereas Dipladenia tends to be smaller, shorter and won't climb unless trained and tied in. More info here https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-a-dipladenia.htm. Otherwise, both these plants prefer full sun, so it may be the difference in conditions is having an impact on growth.