Could you please specify which version of Kentico you are using? I remember there was a problem in versions 6 and 7 where some of the characters in the page alias were overwritten with characters without diacritics when using e.g. Azbuka or Japanese alphabet.
If that is the case, there are basically 2 possible solutions:
1. Utilize OnBeforeRemoveDiacritics event to handle the character rewriting. More details about this can be found in Developer's Guide - Custom handling of URL path values.
2. Upgrade to Kentico 8 where the problem no longer occurs.
Would one of the options work for you?