Press the seam allowances in opposite directions- so that when you put the two right sides together, the allowance of one seam will be aiming in one direction, and the other will be aiming in the other direction. Then they will "nest" together. Then pin them to keep them together. If you're doing a row that has several seams, pin the seams together first, then pin the rest.

Starch is a life saver. But you should starch before you do any cutting.

one of the keys is the ironing of the seams in the 'right' direction, this will help with the nesting of the seams when you sew them...

Are you pressing the seams in different directions. Pressing the seams on one piece one way, pressing the seams the opposite direction on the other piece helps them 'nest'. They will match everytime as long as the pieces have been cut correctly and the seams sewn consistantly. Don't over 'press' your seams.
If seams/pieces are 'ironed' the pieces will sometimes become
misshapen causing the seams not to match as well.

When I iron my seems I iron one row to the left then the 2nd row to the right, repeat the process until you're to the end. When matching the seems the seems will butt up to each other thus matching the seems. After I match the seems I pin them and then pin the rest of the row. Hope this helps

Mine are always off too. I am doing a quilt where it says to press your seams open. No one's ever suggested that before, but all of my seams matched! No bump to go over. I also have a 1/4" foot with a piece of metal that hangs down that my fabric rests against.

I discovered that even tho I pinned carefully and matched the seams, the bottom seam allowance would flip the wrong direction because of the slightly raised edge of the machine. I have it in a cabinet and cant adjust so the edge of the arm of the machine is flush with the edge of the table. So I cut and piece of plastic and taped it so this area will be flat and smooth. This helped. Then I discovered that after I pin I will take a long pin and pin through all layers parallel to the edge to be sure the bottom seams are laying flat. Hope this makes sense.

you might want to check your needle position. i was constantly off "just a hair" over 1/4 inch. decided to check where the needle actually hit the fabric and found i needed to move it over one place to the right. now i have 1/4" seams. (this is on a bernina.)

good luck!

put a little cab of fabric glue at the seam that you want to matc. Works WONDERFUL!!!!!

sorry i meant a little dab of glue