A refrigerant leak can cause a freezer to fail to get cold enough; however, leaks are very uncommon. Also, unlike some automobile air conditioners, it is never necessary to "top off" the refrigerant. The refrigeration system is a closed system and the addition of refrigerant is rarely called for.
Most gases used as refrigerants pose health risks if inhaled. If you suspect a refrigerant leak, provide adequate ventilation and contact a qualified service technician.
Never attempt to chip away ice in your freezer. Even the slightest nick of the cooling coils can result in a very expensive repair or even the total loss of your appliance.
A refrigerant leak can be repaired only by a qualified service technician. If you have ruled out all other possible causes, have a technician inspect your freezer.
In theory, if you can locate the leak, a cold weather epoxy seal could be used to patch the hole. However, these patches will often fail after only a year or two. Once a patch is made, then the system must be recharged with refrigerant. Recharging the system can only be done by a qualified technician.