I've had great experience with the GV-800s. The response about the camera pixelating is right on target.
First go through the history and see what camera(s) are using up huge amounts of space. Then try going to the screen where you check/set the masking on where it does and doesn't detect motion. Top left of that screen you will see a little yellow "running man" image that shows the times the camera is tripping to record.
Adjust the light / contrast back and forth and you will likely be able to see if the camera is pixelating. It may need replaced.
You may find that there is something inframe that is constantly moving, as in a display, flickering florescent lights, anything that it is picking up as movement that you can have moved, repaired, or masked so the camera will ignore it when determining motion.
Hope this helps.
TJWiz