Then they are free to try something else. Many times, children will not give you the real reason up front. It may be that they simply do not enjoy the activity. It could also be that the child had a bad day at the event, feels incompetent, and is embarrassed to go back. Tragically, it is also possible that an adult or an older child is physically or sexually harming your child.
Child molesters do what they can to get in positions of power over children.
They can easily do this by volunteering to be a coach, youth worker, or scout leader. I would never jump to the conclusion that this is why my child was unhappy, but it is a base to cover in seeking a cause. Your children may have lost interest because you do not seem to hold any interest in what they are doing. On the other hand, do not make them pursue an activity just because you 'think' they should like it. All of us will have times where even pleasurable activities require work.