Thanks for writing. Did you move the plant indoors from outside when you repotted it? If so, it may be that the plant is undergoing shock from the change. If this is the case, it's perfectly natural and there's not much you can do at this point. Cutting the plant back a bit to encourage new growth that adapts to your indoor conditions should help it look better faster.
Also: Be sure that the soil dries a bit between waterings. Your mandevilla will likely much less water indoors than it did outside. These plants don't like wet feet --- and will die from being overly wet faster than they'll die from being too dry.
---Justin, Senior Garden Editor, BHG.com