Laura, my recommendation would be to take out all the plain strips and resew. Your pieced strips appear to be accurately sewn.
Once the plain strips are removed, measure the length of all the pieced strips to see if they are of equal length. Hopefully they will be very close. Use this measurement to determine the length of the the plain strips.
The next step takes a little time, but you will be pleased with the results. Follow the suggestions in this post and you won't go wrong.
http://www.quiltingboard.com/t-43921-1.htm
Good luck and share with us the results.