How are you export'ing / import'ing the data?
If you are using the exp/imp utility, you have to make sure the local environment variable NLS_LANG at the source (where exp is done) is set to a characterset compatible with the target (imp) database and also the local environment variable NLS_LANG at the target is set correctly.

The above is to avoid characterset conversions which could corrupt the data.

NLS_LANG has to be set at the highest of the charactersets between source and target.

HTH.