There are mainly three reasons why you are not receiving rewards:
1) Your validator is oversubscribed (nominated by more than 256 people)
Open https://polkadot.js.org/apps/#/staking and check whether your validator is oversubscribed. If your validator is oversubscribed, only the first 256 nominators will receive staking rewards. If you are not among them, you will not receive rewards.
How to fix: Make a new selection and nominate 16 active validators with high ranking, high point and low commission.
2) Your validator is not active
Open https://polkadot.subscan.io/waiting and search for your validator. If you can find your validator, it means that your validator is not active yet.
How to fix: Make a new selection and nominate 16 active validators with high ranking, high point and low commission.
3) Your rewards have been credited to your account, but you haven’t claimed
If the first two do not match your situation, the rewards may have been credited to your account, but you haven’t claimed them.
How to fix: Open DOT Wallet, click on "Staking" and claim your staking rewards.
If you still can't fix this problem, please click "Support and Feedback" in the App to contact us.