The main issue, is that we need to ensure the entire flow fits/works. And its not usually "just the level" its bug fixing, its new features, new mechanics, making sure those new mechanics work with the new level design, omg the new box mechanic lets players get stuck underneath, shit the new revision to bear boss that we thought was going to be the final_final_actualFinal version is able to be cheesed by the new doors we made...
Game dev is hard lol We work on it quite literally every single day and we try and aim for about once every 3 months or so. Never guaranteed, but we do try.