you must use the 45 degree line on your ruler when trimming hst's. or at least make sure the center measurement you want is in the center of your block, like a four patch. watch that and trim one side, well two if you can. then turn the block, replace your ruler watching the degree line or measurements and trim the other two sides. you should have an even block then.
also ck your seam allowance sewing. perhaps you aren't consistent.