Nothing to be done now. You have to be very conscious of seam allowances being exact when you sew the blocks together or you have to take them apart and re sew the ones that are not even. As you have already cut them you are sunk. If you have fabric left then make new pieces and go back and remake the squares that are not correct. In future cut cardboard templates of the finished size of each piece so you can easily test as you seam them together. Quilt making is not for the hasty, it is precise and picky work.

I am not very experienced at quilt making but I've not had this problem before. I have made 24 12" Log Cabin blocks which seemed to measure the correct 12" before I put them together.

Now I see that three of them are bigger than the rest. I cut the three larger ones to size but now it's obvious the outer strips are much narrower. Where did I go wrong?

How can I correct this?