There could be a ton of reasons, but the ones I'll ask for a basis are:
Who do you play and where?
Very important, if you were playing top-tier champions rather than champions you like and enjoy, when they get knocked out of favor by item changes or whatnot, your ranking goes with them.
How many games do you play a day?
You may be on tilt after losing a few times, hell, you can even go on tilt when you win. Remember to be humble and sportsmanlike, and tilt should be cut down to a minimum.
Are you playing every game expecting to win?
THIS is a HUGE mistake most players make. Most champions hover around a 50% win rate. Statistically, You're GOING to lose around 50% of your games. If you go in only to win or only to rank up, you're either A) going to have fun 50% of the time, or B) going to become so angry that you go on hiatus from League.
I highly recommend playing champions you like or enjoy (not just top-tier champions or LCS picks, etc.) a TON, and experimenting a TON with them.
I've theorycrafted a couple of really good off-meta builds for my own main,
, and with some of those, I can directly counter Fizz and Zed (Akali is still trouble, but I have a half decent idea on how to handle her I'll try next time I see her), among others.
And realize that most people in League are only playing to win. You can exploit this to your advantage via rage-inducing BM (playing champions with annoying emotes), or you can try to lighten the lane, and make the game more fun for everybody (Laughing every time YOU miss a skillshot). The key to improving your rank lies in two things. Game Knowledge (when does my champion outdamage X, or, when does X become powerful, so I know to play passively for a while (warn your team about this)), and the knowledge that you can't win everything.
As Double D1 EU player xWhattheMoosex put it so eloquently,
"...I've never learned more about how to play my favorite champions then when I've been behind with no hope of catching up."
Don't give up.
If you're looking for more advice like this, I'd also recommend his channel on Youtube (under the same name).
Good luck, Man.