When searching for Titles or Authors that contain Turkish characters, they are not found if search pattern has the opposed case. For example if the author is "Nefes Zade İbrahim" and the search pattern is "ibrahim" than the result is empty. If the search is "İbrahim" than the record matches and search delivers the good results. The characters that are forcing this problem are "ğĞ, ıI, iİ, şŞ".
Did you use the ICU indexing option ? See : http://wiki.koha-community.org/wiki/Correcting_Search_of_Arabic_records
Seems to be a configuration issue.