M-Tron : Can't find it in Sonar anymore....

One possible explanation of this problem :
 
Sonar tries to load every DLL in your VST paths, but some software includes non-VST/plugin DLLs in their install directories.  iZotope does this for example -- all their plugins are two DLLs one is the VST wrapper and the other is the actual code.  Other plugins include DLL libraries for doing things like loading wav files/etc.  When Sonar loads a DLL  it looks to see if it is a plugin, if it fails, Sonar records that in the registry and then won't load/scan/try that DLL again. 
 
You can force it to re-try everything with the check box in the Plugin Manager's VST Configuration Options panel "Re-scan failed plug-ins" then do a scan with the "Scan VST Plug-ins" button. 
 
Of course this takes forever and a day and it doesn't always fix everything.  The reason why (appears to me, I'm only guessing) is that some plugins fail in epic ways, which then cause the next plugin in the scan to fail.  Even though it may actually be fine.  And since the re-scan option forces everything to re-scan the same double failure happens again.  The only simple solution when this happens is to figure out what the bad plugin is and remove it then repeat the re-scan failed plug-ins scan.
 
I got so sick of this I wrote a Powershell script that removes all the failed VST entries in the registry so I could do a re-scan ONLY failed.  Which cleared up a ton of issues I was having.  This mainly seemed to effect 64-bit plugins.  The 32-bit ones load in a separate process, so when they go bad it usually doesn't effect the next plugin.  The 64-bit ones load in the same process and can mess things up so bad that other good VSTs don't scan correctly.
 
Look in your registry under
HKEY_CURRENT_USER\Software\Cakewalk Music Software\SONAR\Cakewalk VST X64\Inventory
 
There should be an entry for every DLL the Plugin scanner found.  By file path.  Find the one for the M-Tron DLL.  If Sonar doesn't think it's a VST, it will have only a couple of entries and have 0 for isVst and isVst3  it may or may not have isBad set to 1 (which may also be part of the problem it marks things as not a VST but not bad so they don't get re-scanned).
 
The fix is to just delete the entire key for that DLL.  Sonar will then re-scan just it when you start Sonar (if you have it scan on start) or when you do a manual scan.