Need help keeping CS up while roaming

I’m decent at farming in lane, I can usually get 70-80 by 10 minutes if I don’t get zoned out or oppressed really hard. But when I roam or spend any time out of lane, my CS per minute goes way down. I’ve seen people leave lane for some time and keep their CS up but I can’t seem to do it. Is there a method or technique to this that I’m not doing or am I just not last-hitting well enough?