As you loose weight, you need less and less calories to maintain your wait. Not taking into account any extreme activities, you need about 14 Cal per pound per day to maintain your weight. This also means that if you just dropped 20 lbs, you are now burning 300 Cal less. Add to that that you might have increased your daily intake with 100 or 200 Cal without noticing and there there is your former 500 Cal deficit that now turned into an equilibrium.

Anyhow, if currently your weight is remaining constant and you want to loose some more, you need to eat less. Do your own estimate of how many calories you are currently eating, then try to eat 500 Cal less.