This usually happens when you're working with fresh unpacked cards or with very dirty cards, that tend to stick together. First of all, double check your Vacuum Offset value, if it's too large you're applying too much pressure on the pile of cards, a good value usually is 16-18.

If that doesn't help, you can turn on the option Sticky Cards inside the Advanced Settings tab. When enabled, the machine will do several extra movements to let fall any attached card. It will be a little slower but it should solve your issue.