Here are two ideas:
- Survey your customers, including those who left or who are causing problems.
- Automatically analyze / summarize what users post on Twitter and other social networks about your company. Look for shifts in what users are posting now, vs. 3 months ago.This can be performed with very inexpensive / open source tools. We do it with Perl scripts, web crawling and advanced data mining.
The problem could be external (related to what your competitors are doing), and using internal data only to find the root cause, might not work.