Ask HN: How do I reach customers who don't know they have a problem?

I've created an app for the Shopify app store (an ecommerce platform) that provides a customer stock notification service: http://backinstock.info.

The app has been live for about three months, and there has been a slow but steady trickle of registrations and conversions to paid plans. This is good, but now I need to crank it up. The potential market is quite niche, but much bigger than I have reached so far.

After a few attempts at Google Adwords and SEO the problem I keep coming back to is that few people seem to actively search for something like this app.

Customers who have installed the app generally love it. I think the problem is that if you are a successful and profitable online store you are busy selling things rather than googling 'increase shopify sales'.

How can I reach customers who are doing fine, but could be doing better?