Sorry I can't be more forensic about this, but I have a new annoyance whereby from time-to-time the Insert Synth command is unavailable in the Synth Rack. As a workaround, I can go to the Plugins tab of the browser and drag and drop the synth into Track View. After this, the synth in question is visible in the Synth Rack, which then appears to behave as normal (except that Insert Synth ... remains greyed out).
 
Insert Synth ... today was working normally, but then I attempted to select a different layout. Immediately, it became impossible to Insert Synth ... after that and this has persisted through a reboot. This made me suspect a corrupted layout file or something. I have had a look to see if there is anything abnormal-looking in the folder where the plugin layouts are stored but I see nothing unusual. It remains possible to select alternate layouts via the PlugIns tab of the Browser, in any event.
 
Anybody experienced this and/or knows how to fix it?

 
While attempting to understand what you're describing (I've always just double-clicked on the synth name in Browser/Plugins/Instruments), I came across inconsistencies between Sonar, Sonar Help and the Platinum reference manual.  It's a mess and confusing.
 
In the Synth rack, I have no "Insert  Synths" menu item in "Synth Settings Menu" as described in the manual, nor do I have the "Insert Synths Options" menu item as described in Help (greyed out or not).
 
But in any case (and described in both Help and the manual), doesn't the big at the top of the Synth rack do what you want ? 
 
It's called "Insert Synth".  (Or is this what you're saying is greyed out ?)
 

Thanks, Paul. You are correct. The  gives access to Insert Synth ... and to Insert Rewire Device ... . The first is the one I'm talking about, but it's now greyed-out for me. It wasn't, but now it is. I've even run the latest Manchester installer again, but that has made no difference. Your method works for me still, so that's going to have to be the workaround for now.

 
OK, I fully understand now.  Your "Insert Synth" menu item in the list you get after clicking on the "+" is greyed out.  That's odd.
 
I just took some time to learn the Plug-in Layouts functionality (which I'm now going make use of)  and created several layouts in the process.  I used the menu tree in question quite a bit and can't get "Insert Synth" to grey out.  But I'm now scared it will !
 
I'm on Kingston if that makes a difference.
 

Try removing all the entries from the Plug-in Menu Layout folder and Start SONAR. I believe this will default back to the original "Default All Plugins" layout.

Thanks, Steve. I cleared out the folder entirely as you suggested, but this hasn't improved matters. I also then created a new layout (a handful of tried-and-trusted VSTi s) via Plug-in Manager. This makes no difference either (although the new layout is perfectly usable via the PlugIns tab). It's not such a big deal as to make rolling back to an earlier release worthwhile, but it adds to the growing list of annoyances in Sonar, where something used to work but doesn't anymore.

I've had this "bug" for months and months.  Cakewalk support were of no help.  There is another thread talking about it.  All they need to do is explain to me how exactly that Insert Synth menu gets its data.  I could then figure out why it isn't getting it.  But all I got back from them was non-answers to simple questions.  I was thinking I'd try installing Sonar on another computer just so I can dump the registry and see what is different from my main DAW.
 

Ah, that's interesting Matt. Can you by any chance remember at which update it broke? It seemed to break intermittently for me until the other day, after which it seems to have become permanent.

 
It looks like "Insert" / "Soft Synth" in the Sonar main menu and "Insert Soft Synth" in the FX bin use/build the same sub-menu as in the Browser.  Are these not also greyed out  ?
 
The only obvious thing in the registry (there other stuff which I doubt has any significance) is the MenuLayoutsPath but that must be ok if other components and the PM can find the layouts.
 

I believe MenuLayoutsPath  is used to look up where the custom layouts are stored. The registry keys which store the current active menus are in HKEY_CURRENT_USER\SOFTWARE\Cakewalk Music Software\SONAR\Platinum\PluginMenuLayoutMRU

• PluginBrowser
• PluginBrowserAudioEffects
• PluginBrowserSynths
• SynthRackSynths

 
If the default is empty, SONAR uses the "Default All Plug-ins" layout. At least, that is what happens based on my tests.