I have just made the messenger bag (large size), and I too had trouble fitting the back panel. There seemed to be way too much fabric in the back panel. This was odd because, as elleryolivia said, the rest came together beautifully! I think I may have found the problem…..I’ll try to explain. The back panel pattern piece is exactly twice the length of the front panel piece, with the notches in the centre of the back piece. Now on pg 31 of the book, in the section “Attach the back panel and bias binding”, the instructions say to line up the notches of the back panel with the side panel’s gusset seams. But you can see from the pictures on pg 31 that the side panel gusset seams lie below the top edge of the front panel. So this half of back panel, being the same size as the front panel, clearly won’t fit. Are you getting me??!!
Anyhow I trimmed about 1.5″ from the length of the back panel and its lining (keeping the curved corners), so that the notches are now asymmetrical, being closer to the “base” end than the “flap” end. It worked a treat, with the back panel fitting much more easily. Curve clipping still required. I have since made two more bags with the modified pattern (shorter back panel) and they worked great. I’ll put the pictures on flickr. Anyone else had any thoughts about this?!