Some freezers have a timer that defrosts things to make it "frost free". There is usually a switch that lets you turn off the "frost free" control. It will save electricity if it is off.
Most refrigerators control the temperature of the freezer (which is the coldest) and then control the refrigerator section by a device that allows air to circulate between the freezer and the refrigerator sections.
One problem with some is that there is a drain near the evaporator, which is usually behind a panel at the back or bottom of the freezer. Sometimes that drain gets plugged and must be cleaned out.
Another problem is that sometimes the evaporator gets covered with ice which prevent it from operating properly. There are two ways to get the ice off the evaporator; the easy, slow way; and the harder fast way.
The harder fast way is to remove everything from the freezer and get out your screwdriver and remove that panel that covers the evaporator. The, using a hair dryer and your fingers (sharp screwdrivers are not recommended because if you punch a hole in the coil you have ruined the refrigerator) remove all traces of ice in and around the evaporator coils.
While you are at it, make sure the fan at the evaporator is working.
If you find no ice then you have done all of that work for nothing.
The slow and easy way is to remove everything from the freezer and pack it in insulated coolers or boxes with crumpled newspaper for insulation, unplug the refrigerator, and blow warm air into the freezer with a fan (slower) or hair dryer (faster) until you are sure that all of the hidden ice has melted. One problem with this method is that you usually can't get at the maybe-plugged drain.
After you are done, turn the freezer temperature to coldest setting and use the refrigerator control to keep the lettuce from freezing.
If this doesn't fix the problem, and your refrigerator is more than about 10 years old, you should replace it rather than try to get it fixed because the new ones are much more efficient and you will eventually save the cost by using less electricity.