I'm having trouble carrying.

This applies most to when I play Diana. 60-70% of the time I carry pretty hard as Diana being at least 20/10, but I don't know what to do with it. Like my team to be quite honest doesn't keep up. My last game was the biggest example. I was 15/2 at one point and ended up finishing at about 25/13 I think simply because my team wouldn't follow up. I could provide the kills and win most of the team fights, but when my team is doing terrible I don't know how to finish out a game even with that big of a lead. Tips and advice? Don't get me wrong I make mistakes, but I get big enough leads that should just win the game, but for some reason me and my team just don't finish out strong.