If the device cannot achieve the target temperature, there may be problem in the refrigeration cycle.
Check to make sure the heat exchanger is clean of surface dirt and dust, as that is a common cause of such problems. Likewise, defrost the cooling coils in the refrigerator, they can be iced over, which slashes performance.
Also, check the door seals, to be sure they fit properly and that there are no leaks.
If all that is fine, you may need to replenish the working fluid in the unit. There should be a sight gauge that let's you see if the fluid level is low. Note you need the same fluid as is in the unit, mixing different refrigerants can result in damage.
If the fluid is OK and the surfaces clean and the unit is tight, you may have a compressor problem, a more costly thing to fix.
Good luck.