Tips on closing out games?

Hey guys, so lately I been having a lot of trouble finding out how to close out games. I win eventually i guess but i feel like i could be doing so much better if im fed. My last rank match as ahri we were a bit behind early game but coming to mid and late game we took the gold advantage and were able to get some pretty good picks but we werent able to finish out the game. I was able to put pressure in other lanes hoping to force somone away from a team fight so we could 4v5 but it never really happened. Anyone have tips on finishing out a game better. I felt like even tho with a gold lead they were able to make us react to their game play instead of us controling better. We did have good control, we took 4 dragons and a baron while they didnt take any. I know the low elo players including myself like to stall while higher elo players are able to dominate once they have a lead.

Well i usually like to just play mid and play ahri because i think shes my strongest champ and most comfortable with her. I'm able to roam and help other lanes. I also play adc but i feel like if i lose bot lane its hard to roam and help. But i find it when i play mid i cant close out games as easily say if i was playing Jinx. Her siege is great and her atk speed makes it easy to take towers.