There are a few reasons you may not receive a cash back:

Your purchase or the items you purchased wasn't eligible for cash back.
You didn't join Microsoft Cashback or sign-in with an account that is enrolled in the Microsoft Cashback program before making your purchase.
You didn't "activate" the offer when it was presented to you (see "How do I get Cashback? for more details)
You used a coupon code that made your purchase ineligible.
Too much time elapsed between your click to the retailer (from an offer with the Microsoft Cashback blue shopping tag), or since you activated the offer, before you made the purchase.
You clicked another link (from another online ad or offer, from an email, or from a browser extension or shopping tool) to the retailer.
Third-party cookies are disabled on your browser, or a browser plug-in, virus, malware, or other interference blocked cash back activation.
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.