There are a few reasons this can happen.
Your booking was below the minimum spend. Travel credit needs a booking of at least £150, and lounge credit needs at least £20. Below that, no credit is applied and it stays in your account for next time.
It was the wrong credit for that booking. Rewards credit applies to certain experiences. Please check the app for which experiences are eligible for rewards credit.
Your payment was split into two. Hertz sometimes takes a car hire as two separate payments — one when you book, and another at the desk when you pick the car up. Each payment is treated separately, so a £200 hire taken as £120 now and £80 later means neither payment reaches the £150 minimum, and your credit won't come off. If your payment was split like this, get in touch with us. We'll review it.