Cannot withdraw from course in the corses dashboard

Hi.
I don't seem to be able to withdraw from a course. I click the link in the dashboard, I get the message, I click OK, the page changes the URL to something like /?withdraw=143 at the end, but I am still enrolled.
I tried this on different sites, with different themes and I wasn't able to withdraw a single time.
Can you test if this is working on your test site?
Thanks.