I am a high plat player bouncing between plat 1 and 2 constantly, I usually get a really good lead but am unable to convert it into a win as often as I would like. I am looking for someone who could look over my games and give me some tips on what I need to change or do differently. If anyone can help me I will return the favor if they want their games looked at also
Use your lead to control bottom river and monopolise drake early on. Get bottom tower and mid tower asap. Once you have 2-3 drakes, start looking to monopolise top side river and control Baron so you can pressure it any time. If the enemy desperately wants their first drake, trade it for Baron and walk into their base.
If you can't pressure either Baron or drake, have someone split pushing a side lane and siege the mid Tier 2 tower with 4 people (since it's the easiest to kill). If your team is really far ahead, split 1-3-1. Use your gold lead to buy vision (ie early green ward trinket) or deny vision (ie early upgraded sweeper) without sacrificing a lead in combat stats. Buy early elixirs, especially 
if you're a mage. That thing is like a mini-blue buff + the AP of a Blasting Wand for 400g.
Understand how and where to roam, and understand second intentions behind roaming. That is, don't just roam to see if you can get a kill. Think about what you can get after that kill. Drake? A tower? Deep vision? Have a plan. If you can accomplish your second intention by just blowing a Flash or getting a big chunk on an enemy, there's no need to commit to a high-risk, low-reward dive. Just take the chunk and get your objective.
Communicate your power spikes to your team. If you're playing, say, AP Kog and you hit Tear + Luden's + Sorc boots, you're massive. Tell your team to group. Understand when, how, and why you might want to pull the map apart (usually by split pushing and pressuring multiple lanes/objectives) or condense the map (usually by bringing a five-man siege to a tower that has to be answered by a five-man defence).
Understand your team comp vs the enemy team comp, and the win conditions of both. Communicate target selection.
So tldr:
I mean what are you doing with your lead. That would be the biggest thing anyone who was going to give you advice would need to know.
Are you pressuring other lanes when your laner is out of lane or taking advantage of that fact in other ways? Late game you're generally gonna fall off a bit as a mid laner. It's really important that you help the other lanes get some kind of lead on their lanes as well. Vision control, objective, etc.
We need something to go off of. :P
My experience after tilting down from plat 1/2 back to plat 3/4: always be decisive. You need to act on information quickly and efficiently. Do not waste time wandering around or waiting for people to group. For example, if you make a baron call, it has to be done right then when that call is made. You can't wait for your straggling mid to show up or something. The reason is, if you are not decisive in a high plat game, the enemy will be, and they will respond extremely quickly. Then while you are scattered in weak positions, they are together in a strong position, and they will just smash you.
Top causes of losing games while ahead:
indecisive calls not getting deep wards down and allowing the enemy to farm their own jungle with a harder carry than yours getting caught late game
Remember: late game, champion kills are objectives since the death timers are extremely long. Any catch or ambush can lead to a snowball that wins the game. At that stage of the game, if you are unable to have snowballed your gold advantage, note this: A 2 slotted team fighting a 3 slotted team would get wiped out. A 5 slotted team and a 6 slotted team fighting each other in the jungle is still a very uncertain outcome.
If you're playing a support a 
goes a long way in letting you leave a lane completely unattended as you siege another one. I also often build it as 
when I mid-lane or top-lane just for good measure (a skilled being an excellent counter against picks like 
etc., except if the enemy team has any combination of 
[when he activates E] etc., they will give a bad day, due to auto-interrupting the ult). I don't find to be anywhere even near an optimum-choice for support so I would recommend against having a go bot-lane as a support.
I have learned from this thread that I should start adding Elixirs into my strategies.
