I found that when I use the invisible thread it breaks often...use the same type of thread for both the top and the bottom. I love the sulky thread for machine quilting. I also found that you want to make sure the bottom thread is pulled up though the top of the quilt. I have some links on my blog that may help.
http://www.thriftyideastoday.com/201...-quilting.html
Also make sure you have the right needle for the fabric you are using.