Hi Virginia,
You are correct,puckering is one of the biggest problems a lot of ME folk complain about.
It can be caused by several things, 1. that you have under stabilized the fabric 2. that your desired design is very stitch intensive and too much for the fabric, 3. that when you are hooping your project,it has been hooped too tightly or you have pulled or tugged on the fabric.
My best guess is you are under stabilizing it.This is why I always ,always, always, do a test sew for any project. Some say it is a waste of time.others say it is a waste of thread,but trust me doing a test sew is neither. It not only tells me what colors I need to change,but also what stabilizers I need to use,and how stitch intensive the design is.
I do a lot of hoopless embroidery for that very reason as I tend to be a bit over zealous when tightening my set screws. I have not had a problem with puckering since my early ME days,the number one culprit in my experience is under stabilizing. A very nice lady told me one time that you can never over stabilize a project. A project has never failed from over stabilizing,but it has from under stabilizing.
The tricky part is knowing what one to use with what fabric. There is a very good blog on this site about stabilizers. It is informative and I hope helpful to you as well,check out the Avid Embroiderer,she has some very good tips on the how too's of stabilizing.
Hope this helps you out a little any way!
Let us know how you are doing and about your projects .
Judy 
Help me to start each day with a new attitude and plenty of gratitude!