Ok so I haven't played this game in literally a year so I was just testing out some new mechanics in sandbox before I started the career mode. But then I noticed that for some reason I always ran out of fuel before I got into orbit. After a few more tests I realized that My third stage liquid fuel was being drawed into my second stage liquid fuel. I have no idea why this is happening because my decoupler says that it won't allow fuel crossfeed. I've tried fixing it for about 90 minutes and now I think that I'm going to give up on this game because it's kinda a game-breaking bug for me :/.