You are not playing frequently on this account. One of the key factors in matchmaking rating is how frequently you play. When you do not play very often, you are very likely to lose more SR when you lose. You can also possibly gain more SR when you win, but this is unlikely. To see more regular SR adjustments, try sticking to only playing on one account and try to play frequently. (Playing 5 games a week is considered frequent play in my book.)
Finally as a friendly reminder, Medals do not matter when it comes to determining any personal performance stats. Medals compare you to your teammates who each play different roles in the overall team composition. For Platinum and lower tiers, there is a small influence to the SR adjustment at the end of each match that is based on your performance versus other players around your rank in other matches who play the same heroes as you did and determine if you are outperforming the average stats of your general rank. If so you can gain a little more SR when you win games, and you can loss less SR when you lose games. However, in order to truly gain SR, the key is to finding ways to win more games consistently than lose them.
Since this has little to do with technical performance with Overwatch on your PC or console, I recommend posting any additional questions in the Competitive Discussion forum or check out this guide: