this is not uncommen-or the fault of the retailer-
panals are often out of square.
you can either try to block them square- or just cut them square- either method works-
flannel being a looser weave than regular cottons do block a bit easier than other cottons.
those of us who use panals often have found all kinds of (creative) fixes for wonky squares- since it is more the norm than not...even a panal with one large picture is often way out of square===place a square ruler over the picture/block & see if you will (harm) the picture by cutting your squares - square;
steam also (sometimes) helps get them forced into square.
don't dispare- just think creatively-