I pin basted when I first started quilting and I had wrinkles and puckers every where. I kept trying but gave up the pinning. And it was tedious way to baste. I glue my backing to the batting then press it until dry. Then I glue the quilt top on, press until smooth and dry. . I use a paint roller bottle to roll a thin layer of glue on the batting. I never have to check the back for wrinkles or puckers when I'm machine quilting. I use Elmer's School glue and it washes out.

I also was going to suggest you use a walking foot with straight line quilting. When I free motion, I pin baste and then I start quilting in the center. When I have a portion of it done, I will lay out the quilt and redo the pins and straighten the fabric once again. It is an extra step but seems to work for me. Stay positive and don't give up. Eventually it will work better for you. I learned much along my quilting journey.

It's 100% cotton with a polyester batting. I know that's not the best kind to use but this is what Ive been supplied with by the charity for whom I'm making the quilt.

I've turned the speed down as low as it will go and on a test piece this worked fine.

Haven't cleaned the feed dogs though - good suggestion

I'm not surprised it stays in place with two layers of glue. I don't know what that school glue is sorry (I live in the UK)

I'm using a walking foot and haven't had any problems with it previously but I've not attempted anything at this size before.

I know this is really frustrating but I have learned to really take my time when sandwiching. Prior to sandwiching I spray layers with spray baste. I like that much better than quilting around a lot of pins. I tape each diagonal corner first. I tape it taut, but not to the point where it is distorted. This has really helped with ripples on the back.

Does that mean you spray to attach the batting to the backing and also the quilt top to the batting?

May I suggest that you go to one of your elementary (6, 7, 8 year olds) school teachers or the teacher that does art with the children and ask them about the glue that they use that will wash out 100%. Also ask if it is safe for fabrics. Good luck on finding it in the UK

Elmer's school glue is washable glue. In the quilt shops it is called basting glue. You can make your own too. I use the recipe and it works fine. I have a gallon of Elmers so need to use it up. Turn of CC to see the text to write down the recipe.
https://www.youtube.com/watch?v=mVRr...atterboxQuilts I like my quilt to be flat as paper when I'm quilting. Absolutely quilts up flat and rolls up better too.

I hesitated to offer that I only baste with Elmers Washable school glue. I gave up pin basting and thread basting. I don't like spray basting. I use cotton batting and don't dilute the glue, just drizzle from the bottle in a grid. I don't paint it on. It doesn't have to cover every inch of the quilt. I don't iron, because I am never in a hurry and can leave it overnight to dry. It does not gum up the needle and always washes out. I'm about 30 quilts in, biggest is queen sized on a Brother PQ1500. I have never had a pucker with this method, regardless of which quilting method I use.