Sounds like you are stirring it too fast, or too vigorously - that's the only way you are going to get bubbles in the mixture.
Try stirring it slower - you don't need to mix "fast and hard" to get a complete mixture, even for those that start to set quickly.
Also do a search, somebody asked a question about air bubbles a little while ago and there were a bunch of good tips - tapping the ball with a mallet to get the air bubbles to the top, using a lighter (or other heat source - light bulb works great) to get the air bubbles to rise to the top.