Undeliverable parcel

I ordered something from a company in Germany and they shipped it off to me almost 3 weeks ago. But the parcel didn't arrive and so I contacted the company. They replied that it had come back two times because it had been undeliverable. I really don't understand because I receive shippings all the time, and if nobody is in the house they leave a paper slip in my mailbox that I have to pick it up at the post office. But I never received such notification? Has this ever happened to anybody of you? Could it maybe due to the summer staffing of the postal services?