For starters, you could do this test - the error may be in one of three places -
The instructions
Your cutting
Your sewing
Cut three strips of fabric that you are using 1-1/2 inches wide by 3 inches long
Sew them together with your 1/4 inch seam
Press
Measure the finished width of the center strip. It should be I inch.
If too arrow - you might be cutting the strips skimpy - you could move your ruler over a couple of threads to cut a slightly wider strip
Or
You might try Using a slightly narrower seam alllwance
If the center strip ends up being wider than one ince -
You might br cutting your strips too wide or your seam alllwance is too narrow
that might help narrow the problem down.
For some - the definition of a scant 1/4 inch is about 15/64 of an inch. or a thread or two less than 1/4 inch.
Sometimes the problem starts with the cutting - not the seam allowance.