Constructive feedback is critical to the marketing process. Listing agents and sellers want to know if the buyer didn't choose the house for reasons they control or not (such as price & condition vs location). Listing agents can make it easy for busy showing agents by providing automated email feedback requests and not expecting a return call within 20 minutes of the showing. Most agents need minimum 24-48 hours to return feedback requests. Typically, a showing agent is showing 5-10 houses at a go and will not be able to return requests immediately.
Sellers need to keep feedback request expectations in reality. At least half of the showing agents out there will never return a listing agent or seller's request for feedback (see intro paragraph). Approximately a quarter will give generalized non specific feedback and a gracious few will provide constructive feedback.
My rule of thumb for feedback requests is three times via automated email and/or one phone call. After that, it's pointless. The showing agent clearly isn't going to respond and hounding someone for insight that isn't interested in giving it is fruitless and
When your listing agent presents you with feedback remember not to kill the messenger and don't get offended. Listen and determine if the buyer's objections are items you control. If so, consider if the criticisms are valid, such as are you hearing the same feedback repeatedly? Discuss with your listing agent what you can change and control to overcome the various buyer's repeated objections and change it before the next buyer comes along.
One hard truth in this business is that once a prospective home buyer has made up their mind it is very hard to change it. The purchasing of a home is the largest financial and emotional transaction someone will ever make. I've yet to see a buyer go back to a house and give it a second shot after what they considered to be disappointing showing (for whatever reasons). An expert listing agent can predict potential objections from the buyer pool and should be counseling the seller on how to overcome those objections before they list the house for viewing.