Hi,
Thanks for using 3DNA. Your attached PDB file is helpful in locating where the 'problem' is.
The 3DNA web server is doing its job since the middle parts are clearly far off from each other for any sensible pairs. To get what you want, please try the command-line version. First, run 'find_pair' to get a list of base pairs. You need to manually edit the list as desired, and then feed your revised list to 'analyze'. This is the trick that has been repeated many time on the Forum, and mentioned in the 2008 3DNA Nature Protocols paper.
HTH,
Xiang-Jun