You probably ate out the side and there is daylight showing in the "bowed" part. I've got a lot of the same trouble. Finally I started buying new rulers, still my 1/4 inch seams (scant???) are off. I combined Bonnie Hunter's and Eleanor Burns' advice into one axiom that I repeat to myself each time I cut something.
"Put the line on the fabric and give yourself another smidge". Bonnie says put the line on the fabric and Eleanor says to cut it a little bigger. So far I've got 115 4 patches for my En Provence. Yes, I've squared them up and there is a couple of threads that I cut off on some of them.
I changed machines too. I had a free arm Bernina that my neighbor says I was getting wonky on feeding the fabric. So I changed to a Janome flat bed which is 20 inches. I took it off a long arm Grace frame, but it won't fit in my wonderful cabinet. Cut out not big enough. So Yesterday!!! I ordered another cabinet just for that Janome. Now I'm really out of money. That En Provence had better come together!