I agree that your pressing surface may not be hard enough.
Sometimes, when I want a really good press, I place a piece of two-inch maple dowelling under the seam. (I had my stepfather cut the dowel in half lengthwise so it doesn't roll on my ironing board, but you probably don't need to.)
If I want the seam really flat, I press the seam allowances open instead of to the side. In fact, I do this whenever I can, because I like the look on the finished quilt top better. I don't do this if I plan to stitch in the ditches, though.
Is your iron hot enough? I always have my set at the maximum setting, whether or not I use steam (which I sometimes do and sometimes don't.)
Anne in Vancouver, Canada