What is the ruling of Islamic law regarding concealing knowledge? And am I sinful if I do not teach someone who makes a mistake in their prayer or commits an innovation, due to shyness, or if they do not respond to me?

Whoever learns something from the religion of Allah must teach it to those who need it for the soundness of their worship and for enjoining them from evil. This is an obligation related to agreed-upon duties and prohibitions, not to supererogatory acts or recommended deeds. A condition for the obligation of guidance and advice is the expectation of benefit and safety for the advisor.