There are several reasons why you may not have earned a Cashback reward for a certain purchase:

Your purchase or the item you purchased wasn't eligible for a Cashback reward under the offer terms for that particular retailer.
You haven’t enrolled in Microsoft Cashback, or your didn’t sign in with a Cashback-enrolled Microsoft account before making your purchase.
You didn't activate the Cashback offer when it was presented to you (see "How do I earn Cashback rewards?"for more details)
You used a coupon code that made your purchase ineligible to receive Microsoft Cashback.
Too much time elapsed between the time you activated the Cashback offer and the time you completed your purchase.
You clicked another offer link (from another online ad or offer, from an email, or from a browser extension or shopping tool) for that product or retailer that prevented you from earning Microsoft Cashback for your purchase.
Third-party cookies are disabled on your browser, or a browser plug-in, virus, malware, or other interference blocked activation of a particular Cashback offer.
You're using an ad blocker such as uBlock Origin. Pause the ad blocker or add a filter to disable it while shopping.
Your account has been flagged for suspected fraudulent activity or other compliance reasons.