Losing a prospect doesn't mean everything is lost. In fact, the information you gather from a lost prospect can be very insightful.
While most client feedback helps with incremental improvements to your existing offering, the feedback from lost prospects can shine a light on the opportunity to create an adjacent product or the need to do a user interface overhaul as your product might be looking dated or complex.
Remember, clients were happy enough with your product to buy it. The feedback they give is therefore more 'power user' focused or features that will not bring large amounts of new revenue to your company. Obviously, something being requested frequently by clients should most likely be done if there are no workarounds currently in place but don't let the feedback of the loud minority dictate your product roadmap.
Feel free to email me at firstname.lastname@example.org if you would like to find out ways to better capture and engage lost prospects.