I used the inspector to determine that the balls that get stuck were asleep. Not sure why the engine did that though. Found this post that suggests you can check if an object's asleep and apply a force of 0 to wake it up.
https://www.construct.net/en/forum/construct-2/how-do-i-18/prevent-physics-object-104541
I've also noticed some physics objects occasionally sticking in one of my own projects - so will check if they too have gone to sleep and will try to fix similarly.
In the case of the topik strategist he has balls falling on a plane that is not movable according to physical laws, that's why some of them stop and fall asleep to save resources.
He moves these platforms not by physical means, but through sine.
You can tilt them by a small degree then the physical engine will not think that the ball has stopped and when the platform moves away from the action of the non-physical sine, the ball will be in motion.
You can also increase the bounciness of the ball and it will bounce longer and when the platform leaves it will not sleep.
When you mix physics with non-physics, you need to be careful.