Clogged filter (sounds like you already checked this)
Fan is broken / not working (there is a fan that blows across the coils. Without this airflow, the coils will start to frost/freeze up)
You have a refrigerant leak. This will usually result in one part of the coils being frozen while the other size of the coils are clear.
The thermostat on your coils that tells the compressor to turn off is shorted which will cause the compressor to continuously run and result in frozen coils.