Frequently, coaches will design their training sessions so that after a warm up, they do some technical training, then they progress to tactical training (for older ages), and will finish with some form of a scrimmage and then some fitness work.
The problem with this is they are doing fitness work at the end and technical training at the beginning when they are fresh. When they get to the second half of the game, they start to get tired and they lose their technical proficiency. The coaches response is they need to do more fitness at the end of training.
An alternative way at looking at this issue is to do some of the fitness work in the beginning of the training session so that when they are doing their technical (and tactical) training they are already tired so are dealing with the same issues they will be dealing with in a game.
If you can pass, receive, dribble and shoot when you are fresh and unpressured, it doesn't mean you can do the same when you are tired. The only way to be prepared to execute when tired and when under pressure is to train while tired and pressured
This can be accomplished by doing pure fitness work at the beginning of training or, just as effectively (if not more so) doing the training at full speed (the same speed you might play at in a game) realizing you will get tired and have to really focus to execute properly.
The other thing (and what might seem quite obvious to some but few seem to do it) is if you want to do fitness work as part of your training, mix it in throughout the session to help prepare the players for playing while tired.