When putting diamonds together make sure there is a 'tail' hanging over each end---one each piece, when you put the pieces under the pressure foot make sure your needle hits 1/4 inch in and the 'tails' go on each side of the needle.
When sewing the rows together place a needle at the seam 1/4 inch in from edge on 1st row then place the same needle through your next row the same way. You will get an X shape with the seam where the pin is located.
You CAN NOT match your seams at the raw edge of your blocks and row. You HAVE to match where the seam is gonna' be.
Clear as mud? I hope not. I'm sure others will have other suggestions.
Good luck....