ME use just use CPU to render video

Why is it that when I render video (both Premiere Pro and ME) it mostly uses the CPU to render when before it was still rendering using the GPU? I have looked very carefully but it still doesn't work. Although the time issue is just a minor problem, the main problem is that when using CPU to render, the video has very low fps (E.g.: I set the setting to 1080p 60fps but the result is result is only 20fps 720p)

And because my English is very bad, please forgive me if there are any mistakes