Becky, there are a few things I can suggest. I do not recommend using invisible thread in the bobbin. I usually use a thread that matches the backing fabric. Winding invisible thread on a bobbin will create tension and I actually had a plastic bobbin break under the tension. Even if the bobbin doesn't break, the tension of the bobbin would be impossible to adjust.
First and foremost, before beginning any machine quilting project, I do a sample. I make a quilt sandwich of similar weight fabric and batting. If I plan on outlining, then I draw some similar shapes, if stitch in ditch then some straight lines. I clean my machine thoroughly, getting all the thread and fabric dust out of bobbin area, also open the side and make sure dust is out of the thread guides. Put in new needle. I use a universal 80/12 but some swear by the quilting needles. Once this is done, I start my practice. This allows me to check thread tensions, my speed and rhythm. Invisible thread usually needs the top tension lessened slightly. If the top tension too tight, the elasticity will cause the bobbin thread to be pulled up and show on your work.
There are two major problems I see often when quilters complain about problems with the back of their work. One is caused by putting the bobbin in backwards. While facing your machine, the bobbin thread needs to feed off the bottom and to the right. Then fed thru the tension feed to the left. Very often this minor difference will create problems. The second biggest mistake I see is when you move faster than the machine can keep up. Keep your machine at an even speed and slow down your movements of the fabric. It is normal to believe you can rush, but you are not giving the bobbin assembly time to do it's job. You will see this when doing a curve. The bobbin thread will appear tight.
Give these tips a try and I am confident you will be more successful. Let me know if I can be of further help. :D