Popriv,
It has been my experience that this is caused by one fuselage side being stiffer than the other. You have two options;

1. Take apart the bulkheads you have already installed and re-cut and replace one of the sides with a like sheet of balsa. You can have either two stiff sides or two soft sides but they must be close to the same density.

2. I draw a straight line on my building surface that is a little longer than the fuselage length. You can use the plans here if you have a topside view of the fuselage. I temporarily fit the bulkheads and align the fuselage centered over the line/plans and see if they are going to bend approximately the same amount. If that looks ok I glue in the first three bulkheads and then clamp or weight down the front half while I temporarily install the remaining bulkheads and pull together the sides at the tail, being careful to keep the fuselage sides centered on the line previously drawn. If it doesn’t want to align up straight then you can wet the outside of the fuselage side that doesn’t want to bend. This will cause that side to expand and bend a little better. After everything is straight then glue everything in place and clamp everything in place and wait for it to dry.

Good luck and let us know how it works out.