Finishing an arm bar when your opponent has one hand creating space between your leg and their neck and the targetted arm grabbing the bicep trying to hold on

Title is self explanatory, I am learning arm bars and im getting a little better than them but I seem to have a problem when if I dont do it quick enough, they often do this and secure themselves and I just cant secure the arm bar and when I often try pulling they tripod and roll and push me over, any tips?