For accuracy - or getting off course - there are several places to notice:
I start with washing the fabric - so it will be whatever size it is going to be after it is finished - (Some think starch/sizing is necessary - generally, i avoid using it.)
Cutting - are you using the same ruler - and using the same "guideline" for cutting all the time? Some rulers have very thick lines (the old Quilter's Rule rulers with the gridded lines - how was the ruler lined up?)
Seam width - is it consistent? even if it is consistent - is the block ending up being the size intended?
Pressing - little pleats can be pressed in -
Making a test block can make life easier in the long run.