I have been quilting for over a year and a half. I have done several different patterns including 9 patch, HST, circles, etc. My blocks are always square, my quilt is square, my seams are straight. I have been in a class every week since I started. Well, I saw a quilt hanging in my favorite quilt shop - "Strip Therapy 5" book Prism pattern. I wish I had never started this - I've have never had this kind of problem with anything I have ever made! How in the world do you sew these pieces together and get them to line up. I may have 2 or 3 out of 24 or so that line up properly. If anyone out there has made these type quilts, please help. All seams are on diagonals (not 45 degree, not straight). When you line them up, measure and measure and measure and sew - you open it up and guess what one is offset by 1/8 to 1/4 from where it should be - I'm rambling - or maybe babbling like a crazy woman. If I had to sew these all the time - I'd probably never quilt again.

Sewing on the bias like this is tricky. It doesn't take much to stretch a seam line.

I like to starch my fabrics before cutting, it helps stabilize and prevent stretching :D:D:D

Try folding each piece in half, finger crease the center of each and try matching the center marks. They won't look lined up until after you sew them and press them open. Hope this helps!

sorry, too new to be much assistance and a great question - hoping your stress level drops soon!

heavy starch can really be your best friend when dealing with bias edges or a fusable stablizer- you can get tear away's or wash away stablizer-press it to the fabric before cutting then after the project is completed remove it (the wash away you can wait until the quilt is completed)
either starch or stablizer will be a great help.
good luck.