Text search returns results with some keywords, but not others

Question

This pertains to a tours website for Turkey. I've sucessfully
set up a search form, and results page which returns results when
multiple keywords are entered. Everything is working well, except
when certain keywords are used. For example, the table column being
searched for is called long_itinerary, so if anyone is interested
in a certain site or city, it's bound to be in the long itinerary
somewhere.
Here's the problem, by example: When someone searches with
the keywords Antalya, Konya, Ankara, or Adana, for example, results
are returned from the database, and those keywords are in each
returned tour's long itinerary.
But certain keywords return nothing, even when they're in
practically all of the tours, in the long_itinerary column. For
example, Istanbul, Ephesus, and Izmir, Kusadasi and Pergamum return
no results, even though they're there. I've gone through all of the
most likely keywords, and these are the offenders. If, for example,
I search with "Istanbul Ankara," I get results with tours including
Ankara (and Istanbul, but only because the word "Ankara" is in the
itinerary).
First I made sure the Charset values were the same on the
search form page and the results page. Still had the problem. Next,
I went into the database itself, and copied the word "Istanbul"
directly from the long_itinerary column of the database, and pasted
it into the search function's text field, and searched again. Same
problem--no results returned.
I can't see any difference between the keywords which work
correctly, and those which don't, what baffles me is that even when
I copy a keyword directly from the database and paste it in the
search text field, it returns no results!
Well, I'm baffled! Any ideas what might cause this?

Answer 1

Hmm -- one of my customers is a turkish building company
(Kusadasi BTW), and when *they* add some contents to the DB,
they´ll be entering turkish chars. It´s in particular the
turkish "I" which is pretty different, and this is when a search
for keywords containing a western "I" will fail.
Could that be the problem in your case ?