Why do you think 'jna' is the same as 'na'? It is a different letter. Show me one source that says Navijna means new. You just made that up. It is not 'difficult' at all. McGregor's Hindi English dict. has it. Monier-Williams Sanskrit dict. doesn't but it has vi-jna (p.961 column 1). You can't expect a single-volume dictionary to list every compound word like this anyway.