You may need to make your seam allowances "a thread or two" narrower.
Or you may need to cut your pieces "a thread or two" larger.
Or - if you printed off patterns, the printer may be "off" a bit and distorting the pattern. A lot of patterns have a "verification square" to see if it is printing "true."
A "test sample" that I have found effective:
Cut three strips "your" 2 x 5 inches.
Sew them together using "your" 1/4 inch seam
Press like you usually do.
The center strip should "show" 1.5 x 5 inches and the unit "should" measure 5 x 5 inches.
If it does not, then you need to figure out where you need to make adjustments.
Sometimes a "thick" thread will also cause the unit/block to end up small.