I need help,plase!

I'm using texstudio and I obtain the error messages:

reference xxx on page x undefined on input line xx

for some entries. Still, if I start putting \ref{} or \cite{} the window with all keys opens normally and the entries are there. I first thought that this was due to BibTex or the .bst file I am using, but now I noticed that references referred to sections are also not working.

Can anyone help?! Thanks,