I must admit I've been seeing this too, with Superior Drummer 2 routed through a drum map in Sonar Platinum. I thought that maybe the VSTi just hadn't loaded the samples yet, but I have it set to load them all at startup so that shouldn't happen. So, chalk this up as a potential "me too".

I also had been experiencing this problem.  I tried changing the MIDI buffers setting, even as high as 2000 with no success.  It almost seemed like the buffer value was being ignored.  So I emailed support.  I got a reply the next day from Joe in support who recommended deleting the TTSSEG.ini file (it's in the %appdata%/Roaming/Cakewalk/Platinum folder (that's not the exact folder name, but you should be able to figure it out)).  That file contains the MIDI driver settings.  The values in it seemed pretty simple and obvious stuff you could change in the Preferences section.  I don't know how, but deleting it seemed to work.  I still had to up the buffer setting to 1024 (250 worked in X3 on the same machine), but at least now changes in the buffer seem to work.  It's been three days and no dropped MIDI notes.
 
Jim

Jim Polishski - That seems to have done the trick! I only tested it on one project so far, but that was the song with the most problems, so I'm highly optimistic. For the rest of you ...
 
"C:\Users\(your user name)\AppData\Roaming\Cakewalk\SONAR Platinum" is the default location.
"TTSSEQ.ini" is the file name.
 
Just move/delete it and viola, drums! Thanks again for all the input.

This definitely started happening after the upgrade to Platinum.  I too upped my midi buffers to 1000 with no change.  It kind of sounds like it only affects certain people's machines....as others seem to have no problems.
 
I will try that recommendation Jim stated he got from Cake support about deleting the ini file.
 
I'm hoping the next update will have this fixed along with soloing the kit parts.  Meanwhile...I've reverted back to X3 so I can concentrate on my work.  I know the bakers have tweaks to do and are working to get things fixed, so I am patient about the update...especially since X3 works so flawlessly for me.

Are Cakewalk aware of this issue? Do we need a thread in the Problem Reports section?

OK....today I renamed the TTSSEG.ini (instead of deleting it) and opened up a project in Platinum that I'm working on in X3.  For a few minutes I thought it was going to work fine.  Then all of a sudden a snare drum note was dropping and not playing.  I can move the snare note even just a tick and the snare will sound.  But move it back to where it was and no sound.
 
So I'm sticking with X3 until this gets ironed out.

Interestingly enough, for me it's also the snare sound that seems to drop out. I don't know if the problem is intrinsic to that MIDI event, or whether the other drum kit pieces are just played quietly enough and less frequently enough for me to not notice when they are absent.

It's not just snares, I'm getting dropped notes on Kick Drum events.

Ok, I have a project where I can reliably reproduce this now.

1. It persists across restarts of Platinum
2. It persists even though I renamed ttsseq.ini
3. Whether the hit in question fires or not depends on where I start playback. I have a drum clip stretching from bar 203 to 211. If I start playback at 202:01:000 exactly, the hit at 205:01:479 does not trigger. If I start playback from anywhere else, including 202:02:000, or 201:04:000, or anywhere close to 202:01:000 but not exactly on it, it works.
4. It seems unrelated to the quantity of hits, because I am finding that a snare drops out when a kick does not, despite there being the same number of each in the clip and them being at roughly the same times each occurrence (it's a fast metal beat)
5. The 'where I start playback' issue is relative, not absolute: if I move the clips somewhere else in the song and still start playback exactly 1 measure before the clip begins, the problem persists.

There is only one VSTi playing (Superior Drummer 2), and it is going through a drum map where all the notes in are the same as the notes out. (It's just used to show note names in the Piano Roll.)
 

I haven't been as thorough as you in testing, but my impression has always been that the dropped hits always depend on where I start playback from.
 
Your findings reinforce my gut feelings.