there are usually two things that go wrong if the block sizes come up short and that is proper cutting and correct seam allowance. hopefully you have either tape marked a 1/4" seam allowance on your sewing area or you have a 1/4" foot for your machine. i also keep a seam gauge by my machine when sewing to double check my seams. next make sure you cut the pieces correctly. plus there is the OMG scant 1/4" seam, which is just to the inside of the 1/4" seam mark.
double check your work (cutting & seams) and perhaps you will find where you were off. happy block making. i was trying to find the diagram where it shows the scant seam marker one a 1/4" seam foot. i will post if i find what i did with it.
mary