Ice forming on bottom of freezer compartment Samsung RF4287HARS.

If the door was closed properly:
It doesn't defrost

1. Defrost control board, If the defrost function is not working on your refrigerator it could be an issue with the defrost control board. This control board oversees several internal conditions and regulates the activation of the defrost cycle. A faulty board can be the primary reason your refrigeration unit is not transitioning to the defrost cycle. Over time, the buildup of ice on the evaporator coil will prevent the refrigerator and freezer from maintaining proper internal temperature. It is advisable to first check the defrost heater and the defrost thermostat for continuity prior to replacing the defrost control board.
2. Main control board A possible cause for a refrigerator not defrosting, especially on more modern units, is the main control board. The main control board acts as a "brain" for the unit controlling the defrost cycle, compressor, and run time, and essentially acts in place of the defrost timer. The control board can be tested using an ohm meter for functionality and will need to be replaced if bad.
   
3. Defrost timer If the refrigerator is not defrosting the timer could be malfunctioning. Numerous times throughout the day the defrost timer should cycle the defrost heater on as a means to melt frost buildup on the evaporator coils in the freezer. If this timer does not cycle on, then it may be faulty and require replacement.
   
4. Defrost heater assembly If your refrigerator is not defrosting accurately this could be an indication that your defrost heater assembly has failed. This could cause the frost to accrue on the evaporator coils, eventually blocking the airflow resulting in the unit not cooling properly. The defrost heater assembly can be monitored for continuity using an ohm meter and will need to replaced if there is none present.
   
5. Defrost thermostat Another cause of your refrigerator not defrosting is a faulty defrost thermostat. A requirement for the defrost heater to activate to melt away frost on the evaporator coils is a functional defrost thermostat. The defrost thermostat senses the temperature of the evaporator coils and initiates the thawing process by activating the defrost heater. If the thermostat is faulty, it will not sense the lowering temperature of the coils and will not turn on the heater, resulting in an advanced frost build-up. The defrost thermostat can be checked for continuity by using an ohm meter; if there is no continuity the damaged part will need to be replaced.
   
6. Defrost sensor with fuse Another possible cause of a refrigerator that does not defrost properly is a defrost sensor with a fuse. If the defrost sensor fails, the fuse acts as a one-time-only fail safe for the sensor -- if the sensor does not shut off the defrost heater, the fuse will blow when the set temperature is reached. This safety precaution is to keep excessive harm from your unit by the heater and is a one-time use, resulting in the replacement of the entire defrost sensor.
   

1. Clogged or freezing defrost drain If you are experiencing defrost drain issues, it is possibly due to a clogged or freezing drain that is blocking water flow to the bottom of the freezer. This will cause the defrost water to drain and drip down to the bottom of the freezer section. To correct this issue, thaw any ice or remove debris that could be causing the clogged drain or replace the drain heater.
   
2. Drain heater If you are experiencing defrost drain problems, inspect the defrost drain heater. A dislocated or burnt out drain heater will result in the drain tube freezing over. Attempt to return the part to the designated position or replace the damaged part to correct the issue.
   

Dont worry about it, you probably didnt get the freezer drawer closed all the way. frost will form when it doesnt get closed properly. just make sure it is closed now and the unit will clear the frost in a few days thru normal defrosting cycles. be sure that the drawer is closing on all sides by looking at the gasket and all should be ok in the future.