We often forget how many outside factors actually impact the athletes that we see on TV, one of the biggest factors why it is difficult to see a stable progression from junior-to-senior is the unexpected thing that we call life.
Some athletes don't have good training conditions (especially in smaller Biathlon nations, it is easier to win with talent when you are younger, but senior level is all about infrastructure)
Some athletes are full time government supported athletes, other athletes support themselves or their families with additional jobs, leaving them with less energy or less frequent training
Some great athletes are stopped by multiple injuries, they miss seasons of training etc.
Some athletes lose motivation, we have to remember that we're talking about young indecisive people. I am 24 and I still sometimes have no idea what I am doing and here we are talking about 20-22 year olds at university.
Training staff - some training staff can bring the best out of their athletes, some just have a reputation of not producing the best from their athletes. Best example: Sweden under Pichler and how atrociously bad Sweden was without Pichler.
And these are only some of the factors, there are a lot more, because every single journey is different and every single problem affects every individual in different ways.