It could be that the monitor voltage to the glow plug card is not as high as it was prior to changing the glow plugs.
A properly functioning glow plug card will beging to cycle the power to the glow plugs as the voltages increases on the glow plug side of the relay. As the voltage increases on the glow plug side, the controller will cycle the power to reduce the amount of time the plugs are heating. Higher voltages can be from open plugs, bad connections or possibly a higher than normal output from the charging system.
So if your glow plug relay is cycling, one may want to check to see if there are any open glow plugs or check the charging system to ensure that it is supplying the proper voltage. Higher voltages at the plugs will shorten the life of them.
Looking at a schematic of the glow plug controller card itself will show that it is using several inputs to determine the relay closure time. The resistance value from the engine temperature sensor and the voltage to the plugs are two variables that determine the time the relay is closed and if it cycles or not.
One can assume that when defective glow plugs are replaced, this lowered the monitor voltage to the glow plug card causing it to stop cycling the glow plug relay.
The glow plug controller card is smarter than most realize.
Karl