My suggestion is:

1) Use weight heavy enough so that you can feel it in your biceps while maintaining good form.
2) Make sure do it with steady tempo, left n right.
3) Don't jerk/swing the weight from bottom using momentum from your back.
4) Squeeze your bicep at the top for a while to feel the pump but don't squeeze too tight.
5) Set the angle of the arm placement to be low enough to make the exercise more difficult.

If possible, send us a video of you doin both types of curl so we can get a better picture