Why is my cookie not sent with a request?

The answer

The usual causes are a different host, a path that does not match, a Secure cookie on a plain HTTP request, an expired lifetime, a SameSite restriction or a blocked third-party context. Work through them in that order using the browser's developer tools: the network panel shows Set-Cookie on responses and, for each request, which cookies were sent or filtered out, often with a reason. A cross-origin fetch also omits cookies unless credentials are enabled.