In the first place, most fitters aren't comfortable asking for feedback in front of the customer. Even if the job has gone to plan and they know the customer is delighted.
So they will 'forget' to ask, even though you tell them every week how important it is to the company's future growth.
Secondly, even if they ask, the biggest challenge then kicks in when the customer then asks 'so how do we leave you a review?'
Now let's fast forward 10 minutes, and the customer's laptop is now open, and the fitter is explaining how they need a Google account to leave a review, or even if they have a Google account where to go to find the correct place to leave a review.