Hey there,
Universal problem.....the grid on your cutting mat will be incorrect. Use one ruler for width to check for accuracy of the ruler used on width of fabic for strips or squares.
Like a sideways T flipped on the left side.
The other problem is the hand or weight of the materials, if they differ or are used warp or weff or whatever it is called sometimes they will stretch more.
Usually I use a 1/4" foot, if pattern calls for scant quarter, I don't care. If I use a consistant 1/4 all my blocks will be the same size. If you can move your needle
move it one thread with and you will be OK. I am still a beginner and will alway have a zillion things to learn.
Best of the best to you - Cindy