This happened to me too. Thankfully my Roland tech was straight up with me and told me that Roland had a bad batch of cartridges where the tab wasn't popping out.
It is very unlikely to be caused by laying them down flat or nicking them with scissors IMO...
If you have had several cartridges do it, and they were all bought at the same time, this may explain the problem.
Roland has kept very quiet about this. I wonder if it could damage the heads?
Of course, it is possible that in some cases, a damaged sensor on the printer may be causing the problem, but if it only happened a few times from a particular batch purchase, then it is likely to be the dodgey carts. you can see if the tab has popped out or not when u remove it.