OP doesn't specify his FFP, but many carriers are fully automated and when you make a request for missing credit, they can verify that you actually boarded and were not off-loaded on another carrier in seconds.

Sometimes with foreign carriers, the request is forwarded by email or even fax and can take months.

Either way, it is the operating carrier which is paying for your miles, thus a good reason for them to check that you aren't defrauding them.