1- use a more fluid sealer, you can add a drop of liquid to the kerr sealer that you have left from the down pack, the Obtura guttapercha needs a lot of very fluid sealer arriving to mid root so the guttapercha "slides" well into the canal.
2- introduce the sealer in the body and upper third of the root canal.
3- Use the biggest silver tip you can get into that canal, and introduce it as deep as you can.
4- Press the syringe and introduce all the guttapercha in one shot, letting its own pressure push back the syringe in your hand
5- Only use 2 pluggers, first a smaller one to go deeper, push fast and only once, and immediately change to a wider diameter plugger, the widest you can use for that canal to get the maximum cushion... and give it another push. In this one you can give mor than one push.
All the process has to be very fast because the guttapercha solidifies very very fast. And never introduce the guttapercha in 2 times because you think the canal is to deep or too wide, because then you will get voids.
Try it and see.
Jay Marlin in person taught me how to avoid them. Of course in the master thesis I got many of those, because I was packing the whole canal, not only the upper third, where is much easiear to avoid them.
I hope it helps.
Dr. Asunta Garcia Casares