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There could be various reasons for this issue. Some of the common reasons are:

1. Incorrect model binding: Check if the model properties match with the Angular call parameters. Make sure that the names and data types are the same.

2. Unmapped properties: If the incoming JSON has extra properties that do not map to the model, the model binding can fail. You can try ignoring such properties by adding the [JsonIgnore] attribute to the model properties.

3. Missing headers: Check if the required headers, such as Content-Type, are being sent with the Angular call. If not, add the necessary headers to the Angular code.

4. CORS: If the Angular app is running on a different domain than the ASP.NET Core app, you may have to enable Cross-Origin Resource Sharing (CORS) in the ASP.NET Core app.

5. Server-side validation: The null argument might be a sign of a validation error on the server-side. Check the server-side validation logic and make sure that all required fields are being validated correctly.

6. Serialization issues: If the incoming JSON is not structured correctly, the serializer may not be able to deserialize it into the expected object. Check the JSON structure and correct any errors.

By addressing these common issues, you should be able to resolve the issue of receiving a null argument in the ASP.NET Core MVC HTTP post method.