If you see something similar to this:

That means another company card is locked in to the tachograph and you can make an authentication with that company card that is inserted in the smart card reader, but the downloaded tachograph file doesn't contain any activity because another company card was locked in to the tachograph.

You have to insert the companies company card in the tachograph and when you remove it from the tachograph you have to answer no to the question if you want to unlock the company card from the tachograph.