There are many possible explanations and many possible ways to investigate

Possible causes

• The sender mis-typed the receiving address. This is hard to do if the sender scanned a QR code. There are usually checksums built in that make it hard to get a mis-typed address accepted. Nevertheless it is possible

• You gave the sender a different address than the one you think you did. This can happen as many wallets issue a different receiving address for each transaction.

• You accidentally gave the sender the address of a wallet that uses a different crypto-currency than the crypto-currency the sender sent. This is fairly common. It is arguably too easy to confuse Bitcoin cash for Bitcoin.

• The sender didn't pay a sufficient mining fee

• Your wallet is no longer synchronising

• You didn't wait long enough

• Fraud

If you are using a web-wallet where you depend on a third party, there are also a lot of other ways in which things can go wrong. Especially if the provider of the service aggregates transactions. (This is one reason I would prefer to use a standalone wallet).

Ways to investigate.

• Go to a blockchain explorer website and enter the transaction-ID. Do the same for the receiving address. Check that your transaction appears. Check that everything is as you would expect it to be.

• Re-check the address you used, letter by letter. re-check the currency.

• If your wallet is hosted by a third party, raise the issue with their customer support service.