I plant a Mandevilla every year on my patio. This year the plant is doing great, but has no blooms.
If your nighttime temperatures were consistently cool this spring (or if they're still very cool), or if the daytime temperatures have been ridiculously hot, those could be some reasons you don't have blooms on your Mandevilla. Generous watering is required as well. This article may help you: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/getting-mandevilla-to-bloom.htm