To give you the best chance of getting your cashback, we suggest you start a new Quidco shop and follow it through to the payment stage in one sitting.
Retailers don't generally award cashback if the item is reserved and paid for later or reserved and paid for in-store or saved in your basket.
This is because cashback is a referral commission paid to Quidco by the retailer for referring you to make a purchase, so if the retailer can't see that your transaction was made through Quidco, cashback can be declined.