I hope this is not too late to post. But I had the same issue and found a solution to it:
Check if all your controller methods have [http] tag. If they all do and still doesn't work go to step 2
In your configure function ensure that you have app.UseStaticFiles(); If it still doesn't work go to step 3
Uninstall and reinstall swagger. If it doesn't work go to step 4 (Core Only)
If you are using Core Install Microsoft.AspNetCore.StaticFiles and reference it in your project. That is as far as I got and mine started working 题主参考下: