If you are encountering this problem, the most common reasons are:
The Secret Recovery Phrase you currently have is not from the account you are missing.
The account you are missing was a secondary account created under the Secret Recovery Phrase you currently have.
To check if that is the case, use https://danfinlay.github.io/mnemonic-account-generator/ to generate 10 to 100 accounts and check if the missing accounts are in that list.
If it is, you can recreate it in MetaMask by following the instructions here.
The Secret Recovery Phrase you have was originally generated by a different wallet provider.
If you think this could be the case, try restoring your account using those wallet providers to check if you get access to the missing account.
The missing account was imported using a private key.
This is what is known as an imported account. In this case, the only way to access the account is to reimport it using the private key.
The Secret Recovery Phrase was written down wrong or is being entered incorrectly when restoring.
One frequent problem is handwriting (poor handwriting when writing down the Secret Recovery Phrase); another is writing the words in the wrong order. The words must be in the order in which they were originally presented.
The Secret Recovery Phrase is actually restoring the account, but what is missing is the value you hold is in custom tokens that need to be re-added.
You can follow the steps here to re-add them.
To better communicate the importance of never sharing your Seed Phrase, we've changed the name of the phrase to "Secret Recovery Phrase". Learn more here.