maybe what you are doing in games that you lose and have good stats has overall less impact on the game then what you do when your stats suck but you win. unlikely since you say you play entry frag and if you entry frag well you should significantly increase chances of winning for your team, but you never know. maybe your positioning, nades and deaths have a more significant impact on the performance of your team as a whole. most likely its just completely random and unlucky combined with wrong decision making by your team mates. That wont change until you play 5 stack with level 10's that actually understand the game tho. just keep grinding, in a year it will be different xDD
also until you play 5 stack you shouldnt assume a single role the whole game. your team mates wont, your enemies wont, so you shouldnt either. its good to train the aspects of a single role sure, but if you want to win more now you should be able to switch between every role there is at any time to fit the need and utilize your mates the best way possible. Some of that requires some sort of IGL thinking and that can be hard if you dont have enough gamesense and playtime against good players, but the fact that you are thinking about it and assuming roles etc. should mean that in the future you can aquire these skills and be a complete player that wins games for your team.