Exposure can be tricky shooting from a drone where the top half of the video is often very bright (sky) and the bottom half is much darker (ground). If you aim the camera down more and you get more ground, the video becomes brighter so you an see the ground. If it's level and the picture is half sky, it can be darker. Maybe you aimed it more at the ground the first 2 days? Anyway, you can tap the screen in various areas to change the exposure. For example, if it looks dark on your device, you can tap on the ground and the ground lights up nicely. Just be aware it's a balance: doing that might blow out cloud detail in the sky.
You can set a manual exposure but for me, the easiest thing when flying is to tilt the camera down a little so you get about 1/3 sky and 2/3 ground. That seems to give a good balance. If shooting raw photos, I typically tap on the sky so I can get good cloud detail because with raw photos, you can brighten the ground without ruining the sky and get the best of both. That's more difficult to do in an 8 bit/channel video so you need to find a better balance when videoing.
Mike