Well, this question is one every business should be asking themselves. The simple fact is majority of customers drift off due to indifference. Not necessarily a bad experience. Most companies figure once they got a customer, they got a customer. Most companies would be wrong.
I was consulting with a business a couple years ago, creating some ads and strategies. During one of our weekly calls I asked them about their customer list. It was a decent size of 19,000+. I had them go into to their data base and let me know how many have not been around in over a year. They came back with around 7000. I wanted to isolate that grouping and do a campaign custom made for them so I created basically a FBI themed campaign. To lighten the mood I made the 'agent' a kid dressed like a detective holding a magnifying glass.
There was a series of 3 postcards. Within one month after the 3rd card landing they had 1400 customers buy again. 1400 'lost customers'. The reason I used a humorous theme was because it can be uncomfortable for both you and the customer to face "why did you leave!". Knowing that most customers fade away rather than leave I did not even address the possibility of something negative.
All of that said, if I had a list of customers who vanished and expressed dissatisfaction I would have came at them differently. Maybe the same theme, but address how the problems have been fixed.
We did another campaign to the remaining list 3 months later and picked up around 500 more. The moral is they may not be gone, just temporarily misplaced