Top fabric getting out of phase with bottom Sewing Machines Parts and Sales in Los Angeles

I’m sewing a straight seam with two pieces of flannel to make a pillow cover. I’ve made several as gifts with no problem but now the top fabric isn’t going as far under the presser foot as the bottom piece when the machine stitches. After a few inches there is a definite ripple or wave of fabric on the driver side of the foot, so I stop and using a flat tool sort of push the excess under the foot to get things back together. This leaves a wrinkle in the top piece that gets sewn over. Not terrible, but this has really just started happening and I don’t know why. The only thing I am aware of that I am doing differently is spacing the pins further apart along the seam line where I join the pieces. Could this be causing the problem? Would more closely spaced pins ahead of the needle keep fabric more carefully aligned? This is good quality fabric, a relatively new needle with new quality thread. I have not messed with tension or anything else.

And now I am down to my last Christmas gift and have somehow lost my little sewing mind.