That's what's happening? My cerebellum is just not activated?
Brain needs to develop coordination, all development is done when you partake in the activity. You may think about developing specific skill in terms of skill itself like - running, but brain also needs to "learn" how to communicate with its various components.
Problems in the brain or simply lack of certain activities may result in poorly developed communication and certain activities feel difficult or uncoordinated.
This is why active playing, running and tree climbing etc, is so important for child's development - it allows the brain to learn how to efficiently execute and coordinate balance, motor, vision and many other systems and then integrate them in to cohesive and effective behavior.
If child does not develop these skills well, then typically they suffer from dyslexia(problems reading especially aloud), motor coordination, balance and in general they feel "clumsy" or uncoordinated. Susceptibility to motion sickness is also common.
So it is not a matter of activation, it is a matter of training. If you did not train your brain by doing things that involve this areas, then you suffer when performing these activities.
It maybe surprising to know that allot of people do not walk and run properly, there is allot of stuff we do that we may not be doing well and usually it takes decades for poor walking habit to transform in to knee replacement or spine surgery.
Many of these come from the fact that we assume we know how, but we don't do allot of common thins right, this we would never even consider.
Everything we do is a skill, and skills need time and thoughtful practice.