I am no expert, but because no one has responded yet, i'll try to give you some advice.
Try different setups because popping them up isn't the only setup for a double leg takedown. Also remember to keep on circling the opponent by pushing him and pulling him because that helps open up opportunities to shoot.
Also, you don't just have to do a double leg. You can try to set up for a single or a high crotch. If you are better at those, then use those instead of double legs. Just make sure you know how to do double legs though.