I've been quilting for years and can never get a 1/4 inch seam consistant either. So don't feel bad. Unless you are quilting for competition, the lack of a perfect 1/4 inch seam isn't something to worry over. When putting blocks together, you can ease extra fabric or stretch it to make it fit. You just don't see an individual seam when the quilt is finished. If your blocks are waaay off, then start measuring the smaller elements of the block, adjusting the size there before putting the whole block together. If I'm doing RSTs, I always make them 1/4 inch larger, then trim them down to size. Or, like in a log cabin block, I make the last strips that you add a little larger, then trim the block to size. Yes, it takes a little longer and a tiny bit more fabric, but making a quilt is not a race, but something to be enjoyed while doing.