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As a starting point, I would strongly recommend reading over the following blog and knowledge base article. They cover the most common points of friction that Alexa developers encounter with account linking, as well as providing some tips on how to best debug your skill's account linking implementation:

Troubleshooting account linking blog
Account linking debugging knowledge base article

Note that It is important that your skill properly handles the AcceptGrant directive from the Alexa.Authorization interface. If you enable the Send Alexa Events permission for the skill, your Lambda function must handle the AcceptGrant directive. If your skill doesn't handle this directive, account linking fails when the user attempts to enable your skill.

After the "Send Alexa Events" switch is enabled, from that point on your skill's Lambda function will be sent an "Alexa.Authorization" interface "AcceptGrant" directive each time someone attempts account linking to which, your skill's back-end must respond with an "Alexa.Authorization" "AcceptGrant.Response", For details, see Request Access to the Alexa Event Gateway

For more details about implementing account linking for smarthome skills please start Account Linking for Smart Home and Other Domains and Validate and Use Access Tokens in Smart Home, Video, and Meetings Skill Code