I paid air miles for a specific seat on a KLM flight in October this year.
When checking in on-line I found I was assigned to a different seat and nothing else was available. Same when I got to the airport check-in desk, aircraft was full, no change possible.
After the flight I contacted the air miles programme via their webpage, explained what had happened, and they refunded my air miles after I completed an on-line form.
This took about two weeks. The only difficulty in the process was that the on-line form required an 'EMD' number, which looks like an e-ticket number but is something else and I had to send an e-mail message to KLM/Flying Blue to get this as it was not printed on any of my travel documents.