Your problem is clearly in your seam allowances. They're off. What I use is the small ruler below. It's called a "Perfect Piecing Seam Guide," and it measures exactly a scant quarter inch from the center of the needle. All I have to do is (carefully) insert my sewing machine needle into the tiny hole in the center of the ruler, make sure it's straight (I have used a 45 degree angle ruler to make sure, but then, I'm a bit on the Obsessive-Compulsive side) It's hard to get the ruler crooked. Put down your presser foot to hold the ruler in place, and do something to mark the edge. I like to use a self-cut strip of moleskin or heel savers. Then don't forget to check your seams. I use scraps to cut 2" strips, and I cut 3 strips. That's 6 inches, correct? But if you sew together all three strips, that's 2 seams at 1/2" total each, or in other words, you final block should measure
exactly 5 inches. Your center strips should be
exactly 1 1/2". So if it's not absolutely perfect, adjust your seam guide again. I love when it's perfect because everything matches up.
The scant 1/4" seam guide does NOT give you a scant 1/4" seam. It gives you a perfect 1/4" seam because it allows you to account for the bend in the fabric over the seam. I've discovered that with thick material that needs to bend, I have to make the seam allowance a tiny bit smaller because it uses more fabric to bend over the seam. Anyway, this ruler and checking the seams before I start each project, because the fabric itself can alter the seam allowance a tiny bit. If that's the case, I'll find out when I do my test.
https://www.etsy.com/listing/111397285/perfect-piecing-seam-guide-14-inch-ruler?utm_source=google&utm_medium=product_listing_promoted&utm_campaign=supplies_low&gclid=CNPv5rf1ursCFdJcfgodrEkAtQ
I hope this helps, and is clearer than mud!