NVR DS-7616NI-K2 live video lag

hi all,
i am using NVR DS-7616NI-k2 for past two years with 4 camera. Recently added 6 camera because of which the live video lags and in the output video its skips playing few seconds, earlier its was not the problem. In the live video all camera plays only in sub-stream marked 2 . please help me anything which i could do to make the live video smoother and downloaded video without skips seconds of video.

thanks you,
karthi26