It sounds a lot like maybe the sashing and/borders are larger than the blocks. Even though it lies flat on your ironing board...you have pressed it flat. LOL So if it's not the right size for the block then what is happening is you're spreading it flat like a pancake on your ironing board and then when it gets on the frame you see the true size. The wonderful but also evil thing about a frame set up is it forces you to keep your quilt square. It is easier to squish your quilt out flat like a pancake on your DSM - but really look at those quilts you're quilting on the sit down...I bet you there is some distortion happening...on the frame your quilt is going to be much more obvious when it's out of square.
Try making a small sample piece with sashing. Double check the width of each block through the center before cutting and attaching sashing - use that measurement...even if it is different than whatever pattern you might be using. Because we all sew at a slightly different 1/4" depending on how much you are losing in the seam allowance your blocks could be even just 1/4" smaller than they "should" be. A small fix but makes a huge difference in the neatness of your quilt!
Good luck!