Well if the structures are in place that may explain it since Oracle will import the tables in alphabetical order, kinda blows a hole in your R.I. (Referential Integrity)

Now that I think about it at the table level I think Oracle will do the same, create the table, then import the data, one at a time, alphabetically. I'm pretty sure about this, not positive, check the import log file (you do have one, don't you?) and see.

Can you import CONSTRAINTS=N and then import the R.I. later?
OR, if the R.I is in place, spool a file to disable the constraints, import, then re-enable them. You MUST be sure that your data is good however.

MH