Land gives small income despite it's developed. What is happening?
Feudal lords manage castles, townsmen cities and clerge bishoprics, look at Glossary.
Probably the building belongs to the unproper type of owner. If the castle is managed by the bishop, it just won't work properly. So click at the particular land and check it.
How convince people to your ideas?
It is described in chapters: Characters: How to care about human relations? and Riding on a Top: Duke: Election.