I'm getting ahead of myself here because my product hasn't launched yet, but I thought about this and would appreciate the thoughts of this community:
You're launching a great app (or any product). Your research indicated that it fills a need, albeit a niche. Now, having observed the sales for a reasonable time, you see that the sales don't really take off; in fact, after the initial flurry of friends' purchases, the sales don't do anything much. You're understandably disappointed.
How do you decide between putting more money into the product (advertising etc.) versus calling it quits?