I still did not receive my postal mail, sent from Belgium to Korea. What can I do?

More than 3 weeks ago, an A4 envelope with a few documents has been sent from Belgium to Korea. It would take 1 week they said. Unfortunately, it's not sent as registered mail, just normal mail, but that should work as well I guess. I still did not receive anything. I tried to find a solution through different websites, but did not succeed. If it is a customs problem, how to find out? How do they notify me? Anything to pay? Does anybody can give me some guidance? Thanks a lot.I still did not receive my postal mail, sent from Belgium to Korea. What can I do?
I also live in Korea and although I have not had this problem myself a couple people I know have. I dont know what you have in the package, but certain items (some meds, expensive electronics, etc.)customs will stop and hold. I'm not sure what you do to get it, but try contacting customs, or even go to a post office and ask. I would bring a Korean speaker with you when you go.