Fixed timing is everything here. When every test has a known slot and the booking form knows it too, there's no jam at nine in the morning and no empty ramp at three in the afternoon.
The retest is a second appointment that leaks away easily. Created automatically with the result, with its deadline and a reminder, it brings the customer back rather than sending them to the next centre along.
Next year's test is the most predictable revenue there is. A reminder in the right month brings the customer back without any selling at all, and hardly any centre bothers.