Blackberry

I have only tried FMQ a couple of times because I just can't seem to get the backing to look good. This is a baby quilt that I am working on and as you can see the top part of the quilt looks fairly good around the little lamb but if you turn it over it has all of these wrinkles. I had this nice and straight when I started the FMQ so I am wondering what is the problem. I think I would like to do more FMQ if I could figure out how to get the backing looking nice with no big tucks, etc.