Because Canada is processing an estimated 3.5 million ETAs per year (!), there are bound to be lots of people encountering problems with them. Some problems might arise just from the way the form was filled out. Other problems might derive from administrative deficiencies. And still other problems could stem from legitimate sources of inadmissibility to Canada.
You might be able to fix some of the ETA problems of an administrative nature yourself, but because of the huge volume of ETA applications, you should be prepared for delays in sort out things with the government.
Where there are potential "yes" answers to questions that might then trigger human review and requests for more information, you should seek out legal advice from a qualified Canadian lawyer. I'm not urging you to do this because I am one of those lawyers, but rather because of the messes I've seen people get themselves into time and time again trying to sort out issues by themselves, which then become far more time consuming and costly to fix than would have been the case if legal advice had been sought in the first place.