The command-line tools process all entries or modifications in the LDIF input sequentially. The default behavior is to stop processing when the first error occurs. Use the -c option to continue processing all input regardless of any errors. You will see the error condition in the output of the tool.
In addition to the considerations listed above, common errors are:
Not having the appropriate access permission for the operation.
Adding an entry with a DN that already exists in the directory.
Adding an entry below a parent that does not exist.