People don't like confrontations. Unless you invite them to complain they won't. You have to call them, ask them them less than 5 questions to determine if they are satisfied and would use you again. If you hear any hesitation in their voice when you ask if they're satisfied, you ask if there are any areas that didn't turn out as well as they expected. You've pretty much got to beat a complaint out of somebody, but you'll find them.
If you aren't calling your customers back, you have no idea how many complaints you have. Most people will keep quiet about it.
I had a gal wait 4 weeks before calling me about streaks through her whole living room, and she only called me because her friend (An insurance adjuster) told her she had to. She had said we were so nice to her and had such a quick response time and always take care of every detail, she felt guilty calling us back. I thanked her for calling us and fixed the problem.
I've got to get back in the habit of calling people a few days after the job is done.