I'm going to preface what I'm going to say with "ask sensei." That being said....
Make sure uke doesn't have his/her balance when applying the technique.
Or...Use atemi full force. Cause horrible elbow pain. Don't be afraid to dislocate the elbow in the throw. Don't be afraid to bounce uke's head on the mat hard, causing a brain hemmorage. Plow your shoulder into his/her face when throwing.
Second paragraph sound a bit extreme? This is why ukes cooperate. Try and keep uke's arm overextended. If beginners don't cooperate, ask your teacher or explain to them yourself why cooperation is a good idea.
When I was a beginner, I always turned out of the standing kotegaeshi pin. Nage asked sensei about it, and he...demonstrated why turning over was a good idea. Worked for me, and it might work for them.