Hi Dorra,
Ztamps: have to be as close as possible to your rabbit AND the RFID reader's not always where you think it is... Try to 'rub' it on the rabbit's nose, around the rabbit's nose or... elsewhere...
Have you tried with different ztamp:s ?
If all of this fails, you probably should ask the Violet support team : http://help.violet.net/index.php?langue=3&type=1