In that case, talk to someone in the group who is experienced and ask her to verify that the blocks are really accurate. It happens more than you think. Quilt patterns in books and newspapers used to be horribly inaccurate. The easiest way to see if any block is accurate is to redraft it on at least 4 to the inch graph paper. Another tip is to NOT watch the needle while it goes up and down. Watch where the fabric goes under the presser foot at a 1/4 inch mark. You can use a small stack of Post-It Notes or a permanent marker as a guide. Measure from the center of the needle out a 1/4 inch. That's where the cut edge of the fabric should be when it goes under the presser foot. And keep your at least two fingers on your left hand together and hold the left side of the fabric as it moves through the presser foot.
Good luck, we've all been there. It takes time to be accurate, so don't give up.
SandyQuilter