Thank you, everybody. I got a quarter inch foot and put it on - it's a new machine (Janome) so probably not a machine problem. I did wonder about the fabric strips. They are Kaufman roll ups and some have pinked edges and some have straight edges. The way I am doing the pattern, I am always sewing a pinked edge to a straight edge. I wondered if that might be part of the problem. I tried sewing with the pinked edge on top and on the bottom - didn't seem to make any difference.
Before I put the quarter inch foot on, I was using the needle over to the right side (#6) which seemed to give a 1/4" seam allowance if I put the edge of the fabric on the edge of the regular presser foot. Maybe that isn't as accurate as I thought?
I did a couple of seams last night with the new foot - will measure them today in the daylight - after I finish unsewing the worst of the old strip sets.
Thank you again, this site is brilliant.