That response does actually give you the exact reason why this is, but given your response to it I imagine you require a little more detail and explanation than that.
Lp gain/loss is determined by an invisible stat called your Matchmaking Rating or what is commonly referred to as your mmr. Mmr is what determines the level of player you are matched up against and the amount of lp you get is also determined by it as well. The way it determines lp gain is it compares your personal mmr to the standard mmr that your rank holds. Having mmr that is above your current ranking will cause you to be matched up against higher ranked people, but in turn you are rewarded with higher lp per win, with the opposite effect taking place if you have mmr that is lower than the average for your rank.
Allow me to use myself as an example. I am currently gold 5. However I have higher mmr than most gold fives so the people I am matched up against in my games are usually Gold 3 and above including a 1 or 2 plat 5 players per game. Now these players are all a higher rank than me, but I also have a very different ratio of lp gain/loss than you do. When I win I gain 25+ and when I lose I only lose about 10-15 meaning it is quite easy for me to climb.
This is the systems way of putting people into the ranks they actually belong in. People with mmr above their current ranks are able to climb faster, while people with mmr lower than their ranks meet a stalemate until they improve.
There is only 1 way to improve your mmr and that is to win. As long as you win more than you lose your mmr will slowly increase to the point where you should regulate out again to the losing and earning the same amount of lp for a win or loss. My mmr is really high because at the beginging of the season I was averaging a 66% win rate after being placed in silver 3. Since then I have dropped to about a 62% win rate, but I still win much more than I lose meaning my mmr stays high.
I can't tell for sure, but looking at your match history, the general statistics of how many people are in each rank, and the fact that sejuani isn't perma banned at your rank I am guessing you are somewhere in the range of high bronze - low silver. Your mmr however is probably at the level of what the system believes lower ranked players are at. This means you lose much more than you gain and will continue to do so until you start winning more.
Here are a few general tips on how you can improve that I can see from just glancing at a couple of games in your match history. If you would like a more in depth analysis of how you can improve just ask.
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You shouldn't be playing ekko in ranked yet: the dude just came out today. There is no way you have played him enough yet for him to be a viable choice in ranked.
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Place them wards: In that ekko support game you placed the most wards on your team, but the enemy support placed nearly double you ward count. Of an hour long game you can place a lot more than 24.
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Build last whisper on adcs: There is no exception to this rule. On every adc build last whisper as either your third or fourth item (not including boots). There may be a few adcs who don't, but those are extremely rare cases and I can't think of one right now.
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Farming: you cs levels are a bit low considering how long some of these games are going. Your goal should be to literally double what your currently getting
Other than that I can't find to much stuff that can be changed from just a cursory glance. You do seem to have a pretty low death count which is good though it could probably be improved a little bit, but I would just keep working on improving in general and slowly start winning more until your mmr is returned to normal.