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Thank you for your attempts to assist. I found the problem.
My .bib was built automatically by exporting from Google Scholar. In their bib. item form the field 'author' comes after the field 'title', and that makes problem when natbib is used.

The warning I have is:"citation 'xxxx' is undefined on input line ..."
But when I use regular \cite command instead of \citet or \citep, everythin works good! So, I guess, my .bib file is correct, and so is editor. Why should natbib package change?

thank you.
those files are on their place..

Please see below an example (something very basic). There is a .bib file in the same directory. On the output, sometimes I see "?" in the text instead of citation, but there is a list of bibliography in the end . But when I convert the same file once again, I can see citation correct (in the Author ...

hello,

I am using TeXnicCenter, and I have a very strange problem.

Every time I convert the same source file to the PDF, the references are represented in a different (but a wrong!) way. What could be a reason?
Thank you in advance