I tracked winners and losers for each drill, scrimmage, etc and determined a day's winner who did not have to do any conditioning and also used the top five in terms of cumulative as the game starters. 10 of 12 players started a game using this system and the players who were most penalized were those who missed practices. and, the punishment wasn't the coach being mad at them, but their inability to score points while they were not there.
By playing every player in every game, we tended to keep up our intensity because there was not a huge drop-off in talent between 1 and 12. In most seasons, the talent starts fairly close together at practice 1 and the difference between 1 and 12 grows progressively bigger by the week and the game. As the talent difference widens, competitive practices are more difficult and sustaining a level of performance in games when the bench enters is more difficult.
On the other hand, we're talking about kids and kids are naturally inconsistent. What we sometimes equate with effort can be attention or performance balancing out. If you play as well as you possibly can for a half, odds are that you're not going to play as well in the 2nd half.
There are many plausible explanations. As for correcting the problem, I'd say most game issues develop from practice. Do they maintain intensity throughout an entire practice? Are they fatigued during the games? Are they conditioned sufficiently for the pace and duration of the game? Are their skills consistent and automatic or are they still developing skills and are therefore prone to be inconsistent? How do they react to the presence of their parents/fans during a game? I coached a player who was terrific in practice, but she freaked out when she heard her dad yelling from the stands. She could not re-group from a missed shot or turnover because of his voice of displeasure in her head. What kind of mindset? With a fixed mindset, as long as things are going well, it reinforces their beliefs and they play better; but, once they feel adversity, they'd be prone to shrinking from the pressure and trying to protect their performance by not making any mistakes, which we all know will tend to lead to more mistakes.