Session Drummer 3 (x64) not playing all MIDI triggers

I'm using Session Drummer 3 (x64) for the first time.  In the last few days is has been working perfectly, but in the last hour it's started to skip MIDI triggers.  For example, if I solo my Kick drum track, I can see the MIDI notes being played in the output meter of the MIDI track, but not all of them are triggering sounds from the Session Drummer 3 soft synth.  I haven't edited these tracks at all (I imported Smart Loop parts).  It's always the same MIDI notes that are skipped and I've looked at the notes and they are exactly the same as other notes around them that do play.
 
As I say, this drum track was working perfectly until an hour ago and I wasn't doing any changes to the drum tracks when I noticed that notes were being dropped.  I've been working on automation envelopes to control volume and pan on other audio tracks (timing wise, these aren't anywhere near where the notes aren't playing).
 
Any suggestions on what could be causing this and how to fix it would be much appreciated.
 
Thanks

Have you checked the velocities of the offending notes?

They could be set to a really low value and thus, not make any sound.

Also check to make sure they're outputting to the correct Midi channel

Tip: Select ALL of your notes in the PRV by clicking & dragging through the note pane at the left. Now look in the Event Inspector and check the Midi channel. If it shows a number, then all your notes are set to this channel, so this isn't your problem.
If it reads +0 then some notes are sending out on a different channel. Use the scroll buttons to set your Midi channel, I suggest you use Channel 1

I've checked the velocity (all above 100) and channel (all 10) and they seem fine (same as notes around them that are playing). Also, this drum track was working fine for a couple of days, so the only thing I can think of that might have caused an issue is me doing something stupid with the clip automation (I was working on diffenret tracks, but maybe I've done something that's affected the drum tracks).  Any other suggestions?

Just noticed something else weird. The notes that are missed change depending on which measure I start the playback from.

I've changed the output of one of the MIDI tracks to use my external keyboard, and now it's not missing notes, but as soon as I switch it back to a soft synth (in particular, Session Drummer) it misses notes.