Unicode problem and ORA-12899 error!

Hi all,
we had a database which its nls_characterset was WE8MSWIN1252 so we could not save non-Latin data there so we reinstall it with AL32UTF8 characterset.
We run a script that creates all structures and procedures and ....But now when we try to do an insert in a varchar2(30) or char(18) column it raises ORA-12899 error and says the value is too long for this column! we haven't had this before with english data so I think it is because of greater storage for unicode characters.I tried to change NLS_LENGTH_SEMANTICS in system and session level but no chance! same error we see more and more!
I am running 10gR1 on win xp and my nls_languag AERICAN and territory is AMERICA. also ACP is 1252 and OEMCP is 720.
What should i change?
-Thanks very much