Dear Customer… What do you want?

You might run into problems getting honest answers if the sales people they have said no to are the ones that are asking them why they didn't buy. This is likely to be seen by the prospects as pushy sales tactics.

Another option would be to get someone else from the company to follow up the next day with a friendly phone call or send them a survey to answer with an prize or incentive for returning it.

It sounds to me like you need to get out of the building and trying selling these dang things yourself. You need to pitch the product yourself; you need to see when the potential customers' eyes get bigger and when they start to look bored. The information about why people are buying or not buying is way more subtle than anything which commissioned sales staff are going to notice... only the creator of a product can truly have that conversation with a customer.

Once you've done it a few dozen times, you may start to notice very specific trends that you can ask your paid sales staff to watch for. But without experience selling directly you won't know what to ask for.

Not sure what type of product you are selling (since it does make a difference), but unless its something that pretty much sells itself, your salesman won't be able to find the time to ask questions.

Consider your targeting for them to sell "10 units to 5 clients per day", assuming an ambitious 20% success rate, they'll need to meet 25 customers a day to average that figure.

Unless the item they are pushing is easy to understand and requires no explanation or much sales talk, its just not possible.

Lets say he works 8 hours a day minus an hour for lunch and another for "wastage", that leaves him with 6 hours of actual working time. That's asking him to talk to almost one customer every 13 mins, pitch the item, sell it, collect, issue receipts, etc and then FIND another prospect. That's already discounting the transportation time needed, the waiting time while someone opens the door or stuff like prospects on the phone and you having to wait till he's done, etc, etc.

Its really gonna be hard for him to find time to ask question and wait for useful answers within that time frame.

If yours is a product that has a higher rejection rate or requires you to make appointments, etc, you can pretty much forget about it. That's unless you want your salespeople to start telling "white" lies on those feedback forms.

I'd focus on getting feedback from people who do buy it. It will help your staff focus on what is important/works. Not sure how flexible your product is to change the features and functionality anyway. You could be making changes to satisfy one group at the expense of the other. You're going in circles and not growing the customer base.

It's too easy for the sales people to claim the price is too high which reduces your profit but increases their sales rate at your expense.

You may want to get feedback independent of your sales staff.