Got an Answer?
Take your time. The tension you put on the yarn is what prevents the squares from being perfect. Try to be relaxed or in the same state of mind each time you knit.
Are you casting on the same amount of stitches everytime and doing the same amount of rows each time as this can affect the square shape
Was it using the same yarn and needles? You may have changed your tension (Maybe knitted looser as you relaxed into it more).