The Samsung ML-2850 and ML-2851 toner cartridges have chips which sense the amount of toner left in the cartridge.  The cartridges need to have the chips changed out when you refill them so that they are recognized as full. 

There are some sporadic reports from customers who have been able to get the cartridges to work without changing the chips (by blocking the sensor in the printer that reads the chip with tape), but we have not been able to verify this, so our advice remains to change the chips with new replacement chips (which are available by Uni-Kit).  

The starter cartridges may not have the chips, so it is recommended to refill non-starter cartridges.