Why can't I measure my blinds accurately?

I am relatively new to sewing, but I am able to make handbags with no problems, most garments with no problems, and quilts with exact 1/4 inch seams.
But when it comes to making roman blinds and curtain panels, I am a disaster. No matter how many times I measure, my measurements are always off on the final product. I think it is all the shifting that goes on with that amount of fabric. This last project I was super careful, using some of my new quilting skills, but I was still long and a little to short on the width.
Any ideas? I'm killing myself here.
Thanks,
Lyndsey

Hey there. I wish you could have attached a picture. It sounds like your seam allowances are bigger than you think. You could always do a test draft with muslin and if that works or doesn't, you can go from there. If you want to salvage what you have, you could add width and length, then cover the seam with decorative ribbon.

HI!
Do you feel like the fabric is slipping as you're sewing and that is why your final product is uneven?