32bit VSTs have very little in common with 64bit DXis.
Is the Windows up to date?

Windows is updated regularly.

So the plug-ins may have never worked. Windows is up to date. Was SONAR installed as Administrator?

I'm afraid you are asking a question regarding something from years ago, so I can't say. My guess is no. I'm only curious why anything but DXi works.

It may be the DX plug-ins were never properly installed.

Here is another thread from the Sonar forum, that talks about a potential way to get some DXi modules to work:
 
http://forum.cakewalk.com/FindPost/3100550
 
I don't know if it applies or if it work, but worth a read.
 
I never went that route, and just abandoned DXi format components when I moved to 64-bit - been a long time now, so I don't remember much about the above workaround - hope you have some success with it.
 
I will say this - once I made a deliberate decision to completely stop using any non-Cakewalk 32-bit plugins, my system has been SOOOOOOOO stable.  I do miss some of the sounds, but many of the synths and effects were migrated by the companies to 64-bit versions.  (the commercial ones, any way).  It was a bunch of freeware ones I got from the web that were just never ported over to 64-bit, so they are just not used by me anymore.
 
I DO still have a 32-bit X1 install sitting around, in case I have to reference any of the 32-bit plugins, but it never gets used anymore, and I will likely remove it whenever the next full Sonar release is offered.
 
Good luck, (and I hope any of the above is helpful, if not - sorry, my fingers felt like typing) 
 
Bob Bone

I have had great success runing old 32 bit DX plugins via a DX to VST wrapper. I have had no problem running 64 bit DX and DXi plugins too. If they are not programed in a strange way i.e. the GUI or some other proprietary coding they are very stable and work well indeed.  

I'd suggest doing an advanced install of X3 and installing only the DXi synths. Then don't just rescan your plug-ins, reset and rescan them per the link above. Not sure how relevant the reset/rescan is to DXi, but it can't hurt. 

Did you install the DXi synths with X3 or were they already installed from an earlier version of Sonar? If they came from an earlier version I suggest you uninstall and reinstall them. You may have a version that needs an update or a 32 bit version.

Also note that unlike VST moving DXi DLLs can cause problems.

Run the advanced install as Administrator and if a "simple" advanced install does not work, I would move Cyclone, P5antom, Pentagon I, PSYN II and TTS-1 out of the "Shared DXi" folder to get positive feedback the installer actually did something. Not sure anything other than advanced install as Administrator is needed though. Running VST activity like reset/rescan won't hurt but there is nothing to reset or rescan for DXi.