GK420t printer not detecting label size

I have thermal transfer labels with a size of 2 3/4" width by 3/4" height.

I suspect the printer is not detecting the end of each label, because when I press the Feed button it advances 3 labels instead of 1.

The light on the printer is flashing green, every 1.5 seconds, which according to the manual means it is receiving data, but I don't think

it's receiving data because it does this even after a reboot of the PC.

I've double checked the routing of the stock and the ribbon and I'm pretty sure it's correct.

I've uninstalled and re-installed the drivers (Win10, and using "ZebraDesigner driver 32/64 bit. Certified for Windows." from Zebra)

Is there anything else that I can try to narrow down the problem?