Peter
I built a Chester Yawl last winter and also had trouble at this stage. Don't get frustrated at this stage ( I was ready to start trimming planks with a chain saw ) It will go together.
I stiched and took apart several times but once I broke away from the directions it all started to come together. The directions call for stitching the bow together while all 12 planks are loosely stiched. I found no way for that to happen.
I stiched the two #1 palnks together and left the forward couple inches not connected. Leave them flopped open like 2 pieces of bread for a sandwich. Then 1 stiched in the #2 planks loose. At this point I was able to get the bow together and stiched clamped tight.
At this point I used the bow as the referrence and each plank was added one at a time- port side #3 and then starboard #3. Always clamping/stiching at the bow and working aft. Now this is a 2 person job since the entire length of the plank needs to be supported while you work. Some of the planks absolutely need to be muscled into place but now you have the leverage you need. Here is where you'll realize the strength of the scarf joint.! As you leverage each plank into place and stich a bit tighter your beautiful boat will appear.
Once stiched tight as can be, you'll flip it upside down. I "spot welded" the entire lenghth of all the joints and let cure fully before removing the wire. If you don't mind looking like a pansie go to Michael's Craft store Cake decorating dept. They sell Plastic squeeze bottles (frosting decorators) with a nozzle that is perfect for laying in the epoxy along your plank joints.
Good Luck
Ed