N prefix before string in Transact-SQL query
The following articles have some good information on the question. The short answer is just that there's a type mismatch between the unicode column and non-unicode string literal you're using. From the KB article, it looks like omitting the N prefix might still work in some cases, but it would depend on the code page and collation settings of the database. That might explain the change in behavior, if you were previously having success with the no-prefix method.

https://support.microsoft.com/en-us/kb/239530