You can check the .log file in the upload folder to find the error message by searching for the word "rejected" or "error". Here is an example:
Record 1: Rejected - Error on table UL_ACCOUNTS, column PLEDGE_NUMBER.
ORA-01722: invalid number”

The error message tells you the table and column to look for in the control file and the control file will show you where in the data file this field is supposed to be.

If the data is in the expected location and appears to be valid, also check for invisible characters that could be throwing off the alignment. In TextPad you can use "view visible spaces".