Hi and welcome to quilting! Just a warning: It's addictive as all getout!
First, don't assume that precut squares are all perfect...I've found that some aren't.
Second, are you addressing your needle straight on? If your fabric is going toward your needle at an angle, it may not come out with a perfect 1/4" seam.
Third, are you maintaining a steady seam allowance or does it waver? Oftentimes as you start and stop a seam it could waver, so the use of a "sewing bunny" could be helpful. Just sew right onto a scrap of fabric [mine are 1" x 3"] at the end of each block, just like you would when chain piecing. Saves a lot of thread too.
Fourth, you might find that sticking a piece of masking tape onto your bed at perfect right angles to your needle and at a perfect 1/4" from the needle could help