The year next to the title should not have any impact on your ability to add episode for a new season. Even if the series has a start and end year range (e.g. "I Love Lucy" (1951-1957)) and you are trying to add an episode with an air date outside of that range, you just just get a warning.
Can you provide exact examples, i.e. which series are you trying to add new episodes to, and what problem are you encountering?