There are two kinds of EFile errors, error code 184, and error code other than 184. Read carefully here:
If you get error code other than 184, please refer to this document or call CRA helpdesk to find out what the problem is. The reasons most likely are your EFILER ID/password are not matching CRA records; or you are not fully approved to submit EFILE; or your EFILER account is locked etc. Here is the PDF file with all error codes.
If you see error code 184, you will also see some error clue codes in the efile result. please read this FAQ link.