First, thing to understand is that it is not your fault that you can't get the fabric into the corner, or that the edge of the block is not straight, or that the finished blocks are different sizes, etc. Its the "Physics Of The Fabric".
The reason "Sew & Flip" creates problems are as follows:
Focus is on cutting the exact sized triangle to fit in the corner at the very beginning.
The actual act of sewing the material will shrink it by a very small amount [about 1%, depending upon the size of the triangle].
Bending the corner triangle from the position in which it is sewn to the final position creates a curve in both the block and the corner triangle. No matter how much you iron it or pound on it with a mallet that shrinkage is what pulls the fabric from completely filling the corner.
Accomplished quilters have established methods to over come the "Physics Of The Fabric" by using many techniques such as "scant quarter" seams, facing the tools or the sewing machine slightly differently. These techniques vary by the quilter, for example, a scant quarter that works for one quilter may not work for another. These accomplished quilters are so used to the alterations of the techniques that they don't even recognize that they are doing them.
So, instead of putting up with these shortcomings or perfecting work-arounds, do it right, from the start with the Perfect Corner Ruler™.
In the following video Ruth shows how the "Sew 'n Flip" method is used. Note at the end that the block is not square!