I had the same problem. Check http://localhost:XXXX/swagger/v1/swagger.json. If you get any a errors, fix them.
For example, I had an ambiguous route in a base controller class and I got the error: “Ambiguous HTTP method for action. Actions require an explicit HttpMethod binding for Swagger 2.0.”.
If you use base controllers make sure your public methods use the HttpGet/HttpPost/HttpPut/HttpDelete OR Route attributes to avoid ambiguous routes.
Then, also, I had defined both HttpGet(“route”) AND Route(“route”) attributes in the same method, which was the last issue for swagger.