dahua switching resolution

Hi, Camera: IPC-HDW4431C-A-V2 When connecting to the camera web interface and setting the video to e.g. Encode Mode=MJPEG and Resolution=1080P, after a while the camera switch the resolution back to SXGA (1280x1024) or H.264H. The camera is set up to save recordings on a QNap NAS. Why is it...