Random notes not sounding at all?

Welcome to the forum!
This is usually due to too small MIDI buffers.
Double the buffer size at "MIDI buffers - Prepare using xxx ms buffers".
 

Kind of you to post so quickly and sorry I am using an iPhone and it submitted before I had finished typing :-)

I'll look for this in options then I guess and will let you know if it works.

Cheers
Rob

Thanks GH I'll look into that as soon as I can get this working and work out how to make and edit tracks.
That's when things will get serious for me again and I can't wait :-)

All the best
Rob
Ps any interface suggestions are welcome.

My recommendation to get an interface was a solution to the issue.  Perhaps you can find a temporary solution, but then again, going for a permanent fix might be the best way to approach this problem.
 
Currently you are using the built in soundcard chip which doesn't work exceedingly well due to the drivers it must use... normally MME.  Factory cards and MME tend to have very high latency numbers and hence, tend to be a real pain to try to work with regarding full duplex multi-track recording especially when midi synths are involved.
 
Sliding the notes gives it enough time to pick up the start/on midi signal for that note. Previously, it was missing it since it was busy processing other things. I've experienced this issue in other ways. A better interface running ASIO can usually handle the high speed processing to get the notes out to the speakers in real time.  The factory chip can not so it simply drops what it needs to drop to keep up with where it knows it's supposed to be. That's a very simple explanation of what appears to be happening.
 
When I try to run my DAW (MC 4,5,6 Sonar) on the factory chip with my interface disconnected, the latency on the midi is so bad it's practically unusable. I get clicks, pops, stutters, and complete drops. I can plug in the interface, restart the computer and Sonar, and the music plays back flawlessly with zero perceived latency or other issues.
 
A couple of temp fixes you can try.
 
1 adjust the latency and the buffers to see if you can get usable audio
2  try ASIO4ALL ..... it's a free wrapper that fools your computer into thinking it can run ASIO. Some folks claim it's the cat's meow..... and others like me can't get it to work at all.   It's worth a try.  If it does work, it will give you some time to save the money for the interface. But eventually, you want the interface and not A4A. There's always a downside to using wrappers.
 
Hope this helps you