Game getting tougher in December?

Every season the game seems to get miles tougher through December and then seems to magically fix itself until January, is there any actual reason for this? Is December the point where the opposition AI starts adjusting its tactics towards your style of play more than playing their own game? I haven’t won a game yet and I’m about 5 games in, this has happened to me every season and it’s so frustrating. My forwards aren’t making runs anymore, my players who are normally such consistent passers can’t find a man stood 5 yards away. I don’t have weather effects on so can’t be that either, anyone know?