The most likely reason is that the ink is improperly cured! Always make sure that your curing unit is working properly.
First check the temperature of the curing unit and make sure it is set as per instructions mentioned on the Technical Data Sheet of the printed ink.
If the settings are correct and you are still facing this issue, make sure that your curing unit is heating properly to the set temperature. To check this, you can use several appliances to verify the temperature inside of the unit.
If all is proper, you can also decide whether or not the belt speed of your curing unit is way too fast.