Don't feel bad, I have been quilting for several years and sometimes my seams still don't match, when that happens I put a dot of elmers school glue where the seam should match and press the two pieces together with my iron. This works like a charm, it is school glue and it washes out and if you want to move your fabric for any reason just touch it with the iron again and it comes up like it was never glued . Sometimes pins can distort the fabric and cause the seams to be out of alignment even though they look aligned before you sew them together.
Good Luck