On my Janome 6600 after you put on a dual feed foot you have to lift the foot to the highest position and reach to the back and pull down the part that fits into the foot. Clear as mud? Hope this helps.

One thing that I haven't seen mentioned yet is to steam the area that you are quilting. Use your iron to try to shrink the fabric slightly and eliminate any fullness that may be developing in the top layer. I'm doing this now on a quilt that I'm working on that has one block that's a little bigger than it should be and doesn't want to lay completely flat. Ironing it with lots of water and steam seems to be helping. Besides shrinking the block slightly, it is also reactivating the basting spray and helping it adhere better. Good luck

~ C

Something else that may have happened -

The backing was stretched very tightly - and then the backing and top layers were just "smoothed" over it -

When the backing is released, it will cause the upper layers to "pucker up" a little bit and then they will pucker/pleat because there is "more" fabric on top than on the back.