The problem is easy to work out.不定时主动表被动,但为何Sometimes needs aren't easy to be noticed.

The problem is easy to work out.这个时候形容词修饰的是人,不定式修饰的是人的行为.The problem is easy to work out （for sb）,对于谁来讲,问题容易解决.
Sometimes needs aren't easy to be noticed.这个时候形容词修饰的是needs （需求）,不定式修饰的是needs（需求）.Sometimes needs aren't easy to be noticed for itself 对于这个需求来讲,不容易被发觉.