Why are the bonus skilled not strengthened when I strengthen skills?
Are the bonus skills designed to work that way? If yes, why? If no, is there some problem happening to me?
At this time, you will need to refresh the bonus skills separately by entering those skills and practicing from there. :)
At the moment, that is how it is designed. But, if you continue using Duolingo for as long as I have, you'll learn that all of Duolingo is pretty much under permanent development. Things are always changing. :)