A few things can cause this. The most common is that your basket has not reached the voucher's minimum spend once fees are taken out. The prices you actually pay, including any promotional (reduced) prices, all count towards the minimum spend, but the saving itself isn't added on top, so a heavily reduced basket can sit just under the minimum.
Percentage-off vouchers apply to items at their full price only. Vouchers can't be used to pay order fees. If you have checked all of that and the voucher still didn't apply, please reach out to our Customer Care Team, and we'll sort it out for you.