if you're using team builder then it's because team builder uses matchmaking adjustments when you're using a new character in a new role. so whenever you try a new champion in team builder then you're up against other people who are just as inexperience as you are. as you win more games team builder stops matching you up with these "new" players and more with people who are already comfortable with playing their champions and who probably already know how to deal with your champion. so it's not that you're getting worse with your champions, it's that your up against better players.
also as you play your matchmaking adjustment or MMR changes depending on how often you win, so if you keep winning you'll eventually fight against players who are not only better then you are, but also better at the champion that you are using. From experience they know where that particular champion is strong, and they also know where that champion is weak because they too have been crushed by that champions weakness, and since they know they won't hesitate to exploit it. If you do get carried through these games you'll only face better summoners.
also STOP FOCUSING ON KDA!!! that is not the be all end all measuring stick for how well you play a champion.
like most video games we summoners have a skill tree. they are divided into basics skills (skills that apply to all champions), and champion skills(skills related to a specific champion)
the basic skills are
-reaction time(self explanatory),
-last hitting (ability to get gold off minions without retaliation)
-champion knowledge (knowledge of what each champion can do),
-danger sense (admiral akbar's trap alarm),
-vision control (tied in with danger sense, this is your frequency of placed wards, knowledge of where the best ward placements are, and any ability related to the minimap)
-skillshots(your ability to land skillshots and dodge them)
all champion skills are unique to their specific champions but these skills all champions have in their unique ways.
-champion comboes(the most banal of the champion skills. being good at these is probably what most people think constitutes to being a "master" at that champion. while important this only makes up a tenth of how good you are with a champion but is the easiest to grasp)
-champion damage output (exact knowledge of how much damage your champion can do in what instance. it also includes the knowledge of how much damage your opponent can do to you in that same instance. at high mastery levels a summoner can instantly know he can kill an opposing champion at 75% hp and does it. It's basically amounts to a voice that at seemingly random intervals shouts KILLL HIM!! and then the next instance the enemy laner is dead with no oposition in sight)
-champion matchups (this can only be taught through experience, this is how your champion does vs other champions that they have to lane against. It also includes knowledge of what what to build agains what champions. most adcs will rush IE regardless of the opositon but for top laners the choice of first item can make a huge difference in who wins the lane. ie buying ninja tabi vs riven/pantheon)
-zone control(this is knowledge of where you are safe relative to other champions, and where you can exert control of other champions zones, it's also your ability to harass without getting counter harassed. for example a fighter does not want to be 550 units away from a marksmen. the marksmen can hit you without you hitting him. the fighter wanst to be 200 units away from the marksmen where you can both hit each other but your attacks hurt the marksmen more. this sounds simple but with five other enemies to consider and your own allies to protect you/be protected by you it's can get very confusing)
hope that helps. and feel free to point out any skills i missed.
