Most people attribute their losses to pure luck. However, below ~1600, players tend to have blatant weaknesses in certain areas. Let me give a few examples.
I've done 1v1's against people that are ~800 Elo and lost. These people clearly have mastery of the lane phase and have extensive knowledge of champ matchups. However, I tried duoing with one of these people, and the match proceeded somewhat like this: they got ganked multiple times early and never survived a single gank, then when the team started grouping they ran off solo to farm or push, and when we had objective control they decided to base to buy instead of securing the objective. Even though we had a gold and comp advantage, we never fought at the right times and we were effectively down a person at all times, so we lost.
I've also fought a player who was detestable in lane. He had no CS'ing ability, didn't know how to properly trade, and of course he couldn't last hit under his turret. However, he never died because he had a sixth sense about the location of the enemy jungler. Once we got to midgame, he was a beast at controlling objectives and invading without getting caught, and once teamfights started rolling around, he would get multikills. He also followed instructions very well and never complained. Despite being heavily down from the onset, we went on to win a 40+ minute game because everyone else was able to hold him up during the early/midgame.
Now obviously the first player would benefit the most by developing game sense and understand the fundamentals of the game, but he would always complain about "win lane, lose game." The fact is, it doesn't matter how hard he wins his lane; he has no ability to win through attrition in the lategame. He can get fed but doesn't know how to abuse that feed. The second player, on the other hand, is always heavily behind on gold and can never get fed. He relies on his team to win the gold race and hopes that teamfights and objective control can go his way, and if they don't, he's too far behind to attempt to catch up. He would benefit greatly by learning how to CS and properly trade.
Now what's my point in using great detail in these two scenarios? Because one weakness can easily negate five strengths. If you place a pro or someone with good, well-rounded mechanics and knowledge down at 300 Elo, they would climb out with nearly zero trouble by carrying every game until they start getting close to their true Elo. Yes, not every game is winnable (I saw a Twitch back in S1 that was ~270 wins and ~13 losses back when dodging was a 10 Elo penalty), but the majority of games can be turned around if a few things are done differently.
If you truly believe that you're playing at a pro level, and you're not seeing the same results as a pro, then you have to break that mental barricade before you start really improving. The sad fact is that most people who come to the forums to complain aren't seeking self-improvement but instead are just here to vent their frustrations, upvote people who agree with them, and downvote anyone who disagrees with them.