Hi everybody! I hope someone can help me :(

I have Windows 7 on my laptop and Windows Movie Maker. I've been trying to make a video for the past three days and had to de-film them because each time I finish editing, and finalise the video, the video is then darkened. I've changed the brightness both on my Sony A5000 camera and tried brightening up the video in WMM but still, nothing has changed. 

The lighting in my room isn't amazing but on the camera and when I edit, it looks fine. Does anybody have any tips or suggestions? :( I'm so frustrated!